How to fetch a model's relationship in a dynamic way? - c#

So I have an application that makes use of a SQLite database. I have some methods for fetching models out of my database. I have a repository that communicaties with my database. And it looks like this:
public class PhotoRepository
{
AppDatabase db = null;
public PhotoRepository ()
{
db = new AppDatabase (Constants.DatabaseFilePath);
}
public Photo GetPhoto(int id)
{
return db.GetItem<Photo>(id);
}
public IEnumerable<Photo> GetPhotos (int album_id)
{
return db.GetItems<Photo>().Where( x => x.Album_Id == album_id );
}
public int SavePhoto (Photo item)
{
return db.SaveItem<Photo>(item);
}
public int DeletePhoto(int id)
{
return db.DeleteItem<Photo>(id);
}
}
I am concerned about the GetPhotos(int album_id) method, because the method it approaches in my database class looks like the following:
public IEnumerable<T> GetItems<T> () where T : BL.Contracts.IBusinessEntity, new ()
{
lock (locker) {
return (from i in Table<T> () select i).ToList ();
}
}
As you can see, it retrieves ALL of the Photo Models from my database and after that I'm, filtering those out that corresponds to the right album_id.
No my question is:
How can I rewrite my method so that it ONLY fetches out the Models corresponding to the album_id. Or even better, how can I rewrite this method to filter any relationship on any property?


So I managed to solve my own answer. Maybe people are interested in the result, so here it comes.
I read a post about dynamic LINQ queries. The post I am talking about can be found here. So the first thing i did was making a Filter class that looks like this:
public class Filter
{
public enum Op
{
Equals,
GreaterThan,
LessThan,
GreaterThanOrEqual,
LessThanOrEqual,
Contains,
StartsWith,
EndsWith
}
public string PropertyName { get ; set ; }
public Op Operation { get ; set ; }
public object Value { get ; set ; }
}
Then I made the ExpressionBuilder class that looks like the following:
public static class ExpressionBuilder
{
private static MethodInfo containsMethod = typeof(string).GetMethod("Contains" );
private static MethodInfo startsWithMethod =
typeof(string).GetMethod("StartsWith", new Type [] {typeof(string)});
private static MethodInfo endsWithMethod =
typeof(string).GetMethod("EndsWith", new Type [] { typeof(string)});
public static Expression<Func<T,
bool >> GetExpression<T>(IList<Filter> filters)
{
if (filters.Count == 0)
return null ;
ParameterExpression param = Expression.Parameter(typeof (T), "t" );
Expression exp = null ;
if (filters.Count == 1)
exp = GetExpression<T>(param, filters[0]);
else if (filters.Count == 2)
exp = GetExpression<T>(param, filters[0], filters[1]);
else
{
while (filters.Count > 0)
{
var f1 = filters[0];
var f2 = filters[1];
if (exp == null )
exp = GetExpression<T>(param, filters[0], filters[1]);
else
exp = Expression.AndAlso(exp, GetExpression<T>(param, filters[0], filters[1]));
filters.Remove(f1);
filters.Remove(f2);
if (filters.Count == 1)
{
exp = Expression .AndAlso(exp, GetExpression<T>(param, filters[0]));
filters.RemoveAt(0);
}
}
}
return Expression.Lambda<Func<T, bool>>(exp, param);
}
private static Expression GetExpression<T>(ParameterExpression param, Filter filter)
{
MemberExpression member = Expression.Property(param, filter.PropertyName);
ConstantExpression constant = Expression.Constant(filter.Value);
switch (filter.Operation)
{
case PhotoWapp.DL.Filter.Op.Equals:
return Expression.Equal(member, constant);
case PhotoWapp.DL.Filter.Op.GreaterThan:
return Expression.GreaterThan(member, constant);
case PhotoWapp.DL.Filter.Op.GreaterThanOrEqual:
return Expression.GreaterThanOrEqual(member, constant);
case PhotoWapp.DL.Filter.Op.LessThan:
return Expression.LessThan(member, constant);
case PhotoWapp.DL.Filter.Op.LessThanOrEqual:
return Expression.LessThanOrEqual(member, constant);
case PhotoWapp.DL.Filter.Op.Contains:
return Expression.Call(member, containsMethod, constant);
case PhotoWapp.DL.Filter.Op.StartsWith:
return Expression.Call(member, startsWithMethod, constant);
case PhotoWapp.DL.Filter.Op.EndsWith:
return Expression.Call(member, endsWithMethod, constant);
}
return null ;
}
private static BinaryExpression GetExpression<T>
(ParameterExpression param, Filter filter1, Filter filter2)
{
Expression bin1 = GetExpression<T>(param, filter1);
Expression bin2 = GetExpression<T>(param, filter2);
return Expression.AndAlso(bin1, bin2);
}
}
Then I added the method GetItems<T>(List<Filter> filters) that looks like the following:
public IEnumerable<T> GetItems<T> (List<Filter> filters) where T : BL.Contracts.IBusinessEntity, new ()
{
lock (locker) {
var deleg = ExpressionBuilder.GetExpression<T> (filters).Compile ();
return (from i in Table<T>().Where(deleg) select i).ToList();
}
}
And to retrieve the records I need, I used the following code.
public IEnumerable<Photo> GetPhotos (int album_id)
{
List<Filter> filters = new List<Filter> () {
new Filter{
PropertyName = "Album_Id",
Operation = Filter.Op.Equals,
Value = album_id
}
};
return db.GetItems<Photo>(filters);
}
Now I can filter nicely on all properties I need! The nice part about this solution is that I can use any class and property to filter on.
I hope this is useful to somebody!

Related

Define a generic lambda in C# and assign it to a variable to be used with MakeGenericMethod

I know I can define a generic function and then call it with Type parameters by help of reflection. Something like that:
private static void SomeFunc<Tf>()
{
// Something type safe against Tf
}
public void CallingFunc()
{
var someType = typeof(whatever); // This may be retrieved using reflection as well instead
var someMethod = this.GetType().GetMethod(nameof(SomeFunc)), BindingFlags.Static | BindingFlags.NonPublic);
var typedMethod = someMethod.MakeGenericMethod(someType);
typedMethod.Invoke(null, null);
}
Now, is there a way to declare this SomeMethod<Tf>() inline as a lambda, to avoid declaring it as a separate method in the class, but still being able to use it with MakeGenericMethod? Something like:
public void CallingFunc()
{
var someType = typeof(whatever); // This may be retrieved using reflection as well instead
// This obviously doesn't work as it requires Tf to be known at this stage, and we can have Tf only as a Type variable here => what should I do instead?
var someMethod = new Action<Tf>(() => /* Something type safe against Tf */).Method;
var typedMethod = someMethod.MakeGenericMethod(someType);
typedMethod.Invoke(null, null);
}
Making CallingFunc generic as well is not an option - the someType variable is retrieved with reflection so is not known on compile time.

Based on: https://gist.github.com/afreeland/6733381 thanks !
With a little improve:
public class ExpressionBuilder
{
// Define some of our default filtering options
private static MethodInfo containsMethod = typeof(string).GetMethod("Contains", new[] { typeof(string) });
private static MethodInfo startsWithMethod = typeof(string).GetMethod("StartsWith", new[] { typeof(string) });
private static MethodInfo endsWithMethod = typeof(string).GetMethod("EndsWith", new[] { typeof(string) });
public static Expression<Func<T, bool>> GetExpression(List<GridHelper.Filter> filters)
{
// No filters passed in #KickIT
if (filters.Count == 0)
return null;
// Create the parameter for the ObjectType (typically the 'x' in your expression (x => 'x')
// The "parm" string is used strictly for debugging purposes
ParameterExpression param = Expression.Parameter(typeof(T), "parm");
// Store the result of a calculated Expression
Expression exp = null;
if (filters.Count == 1)
exp = GetExpression<T>(param, filters[0]); // Create expression from a single instance
else if (filters.Count == 2)
exp = GetExpression<T>(param, filters[0], filters[1]); // Create expression that utilizes AndAlso mentality
else
{
// Loop through filters until we have created an expression for each
while (filters.Count > 0)
{
// Grab initial filters remaining in our List
var f1 = filters[0];
var f2 = filters[1];
// Check if we have already set our Expression
if (exp == null)
exp = GetExpression<T>(param, filters[0], filters[1]); // First iteration through our filters
else
exp = Expression.AndAlso(exp, GetExpression<T>(param, filters[0], filters[1])); // Add to our existing expression
filters.Remove(f1);
filters.Remove(f2);
// Odd number, handle this seperately
if (filters.Count == 1)
{
// Pass in our existing expression and our newly created expression from our last remaining filter
exp = Expression.AndAlso(exp, GetExpression<T>(param, filters[0]));
// Remove filter to break out of while loop
filters.RemoveAt(0);
}
}
}
return Expression.Lambda<Func<T, bool>>(exp, param);
}
private static Expression GetExpression<T>(ParameterExpression param, GridHelper.Filter filter)
{
// The member you want to evaluate (x => x.FirstName)
MemberExpression member = Expression.Property(param, filter.PropertyName);
var typeOfValue = filter.Value.GetType();
// The value you want to evaluate
ConstantExpression constant = Expression.Constant(filter.Value, typeOfValue);
// Determine how we want to apply the expression
switch (filter.Operator)
{
case GridHelper.Operator.Equals:
return Expression.Equal(member, constant);
case GridHelper.Operator.NotEqual:
return Expression.Equal(member, constant);
case GridHelper.Operator.Contains:
return Expression.Call(member, containsMethod, constant);
case GridHelper.Operator.GreaterThan:
return Expression.GreaterThan(member, constant);
case GridHelper.Operator.GreaterThanOrEqual:
return Expression.GreaterThanOrEqual(member, constant);
case GridHelper.Operator.LessThan:
return Expression.LessThan(member, constant);
case GridHelper.Operator.LessThanOrEqualTo:
return Expression.LessThanOrEqual(member, constant);
case GridHelper.Operator.StartsWith:
return Expression.Call(member, startsWithMethod, constant);
case GridHelper.Operator.EndsWith:
return Expression.Call(member, endsWithMethod, constant);
}
return null;
}
private static BinaryExpression GetExpression<T>(ParameterExpression param, GridHelper.Filter filter1, GridHelper.Filter filter2)
{
Expression result1 = GetExpression<T>(param, filter1);
Expression result2 = GetExpression<T>(param, filter2);
return Expression.AndAlso(result1, result2);
}
}
// Filter class
public class GridHelper
{
public enum Operator
{
Contains,
GreaterThan,
GreaterThanOrEqual,
LessThan,
LessThanOrEqualTo,
StartsWith,
EndsWith,
Equals,
NotEqual,
Or
}
public class Filter
{
public string PropertyName { get; set; }
public object? Value { get; set; }
public Operator Operator { get; set; }
}
}

Linq query : Translate local variable reference

I have a situation where I need to translate the local variables reference in a linq query to its values.
For this I'm using this class to do a translate local variable reference in a query :
public static class Evaluator
{
public static Expression PartialEval(Expression expression, Func<Expression, bool> fnCanBeEvaluated)
{
return new SubtreeEvaluator(new Nominator(fnCanBeEvaluated).Nominate(expression)).Eval(expression);
}
public static Expression PartialEval(Expression expression)
{
return PartialEval(expression, Evaluator.CanBeEvaluatedLocally);
}
private static bool CanBeEvaluatedLocally(Expression expression)
{
return expression.NodeType != ExpressionType.Parameter;
}
class SubtreeEvaluator: ExpressionVisitor
{
HashSet<Expression> candidates;
internal SubtreeEvaluator(HashSet<Expression> candidates)
{
this.candidates = candidates;
}
internal Expression Eval(Expression exp)
{
return this.Visit(exp);
}
protected override Expression Visit(Expression exp)
{
if (exp == null)
{
return null;
}
if (this.candidates.Contains(exp))
{
return this.Evaluate(exp);
}
return base.Visit(exp);
}
private Expression Evaluate(Expression e)
{
if (e.NodeType == ExpressionType.Constant)
{
return e;
}
LambdaExpression lambda = Expression.Lambda(e);
Delegate fn = lambda.Compile();
return Expression.Constant(fn.DynamicInvoke(null), e.Type);
}
}
class Nominator : ExpressionVisitor
{
Func<Expression, bool> fnCanBeEvaluated;
HashSet<Expression> candidates;
bool cannotBeEvaluated;
internal Nominator(Func<Expression, bool> fnCanBeEvaluated)
{
this.fnCanBeEvaluated = fnCanBeEvaluated;
}
internal HashSet<Expression> Nominate(Expression expression)
{
this.candidates = new HashSet<Expression>();
this.Visit(expression);
return this.candidates;
}
protected override Expression Visit(Expression expression)
{
if (expression != null)
{
bool saveCannotBeEvaluated = this.cannotBeEvaluated;
this.cannotBeEvaluated = false;
base.Visit(expression);
if (!this.cannotBeEvaluated)
{
if (this.fnCanBeEvaluated(expression))
{
this.candidates.Add(expression);
}
else
{
this.cannotBeEvaluated = true;
}
}
this.cannotBeEvaluated |= saveCannotBeEvaluated;
}
return expression;
}
}
}
And after to translate local variable reference , is used like this :
public class DbQueryProvider : QueryProvider
{
// …
private string Translate(Expression expression)
{
expression = Evaluator.PartialEval(expression);
return new QueryTranslator().Translate(expression);
}
}
For example if I have this expression :
string ct = "London";
var query = db.Customers.Where(c => c.City == ct);
the returned expression after translation will be :
SELECT * FROM (SELECT * FROM Customers) AS T WHERE (City = 'London')
The code is working ok , with all the types of variables , except of DateTime.
In my database I save only the Date part ( without time ) , for example 02/03/2012.
Now if I have this query :
DateTime dt1 = Convert.ToDateTime("01/01/2012");
var query = db.Orders.Where(c => c.dt == dt1);
If I try to translate , the returned expression is :
SELECT * FROM (SELECT * FROM Orders) AS T WHERE (dt = '01/01/2012 12:00:00 AM')
As you can see the translated query contains the time at the end , and my query everytime return no records.
What can I do ?

var query = db.Orders.Where(c => c.dt == dt1.ToString("MM/dd/yyyy"));

Generic Multi-Level Filter for WebApi in C# using Expression Tree, Issue with Nullable Sub Properties

I am making a generic multi-level filter for my WebApi in C#. However i keep getting the following error:
Expression of type 'System.Void' cannot be used for return type 'System.Boolean'
I believe the error happens when a sub property is nullable. For exemple, if i have Entity.SubEntity.description and i filter on description when SubEntity is null, then the error happens. However if the SubEntity is not null, then it works fine.
Other important information:
For the filterDto, the value property can either be an array or a
string.
The error happens when i convert Queryable.ToList() after the return of the Filter function.
Order of calls Filter() -> CompileFilter() -> CompileComparison()
I am not that good with Expression Trees, so i know i am missing some notions here. Any help would be greatly appreciated.
Code Snippet:
public partial class FilterDto
{
public string property { get; set; }
public dynamic value { get; set; }
public string #operator { get; set; }
public bool exactMatch { get; set; }
}
public IQueryable<TEntity> Filter(IQueryable<TEntity> queryable, ICollection<FilterDto> filterDtoCollection, Expression<Func<TEntity, object>> filterProperty)
{
if ((filterDtoCollection == null) || (filterDtoCollection.Count <= 0))
return queryable;
Expression filterExpression = null;
ParameterExpression pe = Expression.Parameter(typeof(TEntity), "x");
Expression ie = GetDeepPropertyExpression(pe, filterProperty.GetMemberName());
Type includePropertyType = filterProperty.Body.Type;
var comparisons = filterDtoCollection.Where(x => includePropertyType.GetProperty(x.property) != null).Select(x => CompileFilter(Expression.Property(ie, x.property), x));
if (comparisons != null && comparisons.Count() > 0)
filterExpression = comparisons.Aggregate((x, y) => Expression.AndAlso(x, y));
else
return queryable;
Func<TEntity, Boolean> expression = Expression.Lambda<Func<TEntity, Boolean>>(filterExpression, pe).Compile();
return queryable.Where(expression).AsQueryable();
}
private Expression CompileFilter(Expression property, FilterDto filterDto)
{
Expression filterExpression = null;
Expression comparison = null;
if (filterDto.value.GetType() != typeof(JArray))
{
filterExpression = CompileComparison(property, filterDto.value, filterDto.#operator, filterDto.exactMatch);
}
else
{
foreach (dynamic filterValue in filterDto.value)
{
dynamic value = Convert.ChangeType(filterValue.Value, property.Type);
comparison = CompileComparison(property, value, filterDto.#operator, filterDto.exactMatch);
if (filterExpression == null)
{
filterExpression = comparison;
}
else
{
if (filterDto.#operator == "in" || filterDto.#operator == "like")
filterExpression = Expression.OrElse(filterExpression, comparison);
else
filterExpression = Expression.AndAlso(filterExpression, comparison);
}
}
}
return filterExpression;
}
private Expression CompileComparison(Expression property, dynamic value, string #operator, bool exactMatch)
{
Expression comparison = null;
Expression constant = Expression.Convert(Expression.Constant(value), property.Type);
MethodInfo method = null;
switch (#operator)
{
case "<":
comparison = Expression.LessThan(property, constant);
break;
case "<=":
comparison = Expression.LessThanOrEqual(property, constant);
break;
case "=":
comparison = Expression.Equal(property, constant);
break;
case ">=":
comparison = Expression.GreaterThanOrEqual(property, constant);
break;
case ">":
comparison = Expression.GreaterThan(property, constant);
break;
case "!=":
comparison = Expression.NotEqual(property, constant);
break;
case "in":
comparison = Expression.Equal(property, constant);
break;
case "like":
if (value.GetType() == typeof(String))
{
method = typeof(ExtensionMethods).GetMethod("Contains");
comparison = Expression.Call(null, method, property, Expression.Constant(value), Expression.Constant(StringComparison.OrdinalIgnoreCase));
}
else
{
comparison = Expression.Equal(property, constant);
}
break;
default:
if (!exactMatch && value.GetType() == typeof(String))
{
method = typeof(ExtensionMethods).GetMethod("Contains");
comparison = Expression.Call(null, method, property, Expression.Constant(value.ToString()), Expression.Constant(StringComparison.OrdinalIgnoreCase));
}
else
{
comparison = Expression.Equal(property, constant);
}
break;
}
return comparison;
}
private Expression GetDeepPropertyExpression(Expression initialInstance, IList<string> property)
{
Expression result = null;
foreach (string propertyName in property)
{
Expression instance = result;
if (instance == null)
instance = initialInstance;
result = Expression.Property(instance, propertyName);
}
return result;
}

Generic Expression tree with 'OR' clause for each supplied property

I have created a generic search extension method for IQueryable that enables you to search for a single property to see if a search term is contained within it.
http://jnye.co/Posts/6/c%23-generic-search-extension-method-for-iqueryable
I now want to enable the user to select multiple properties to search within each, matching if any property contains the text.
The code:
The user enters the following code to perform this search:
string searchTerm = "Essex";
context.Clubs.Search(searchTerm, club => club.Name, club => club.County)
//Note: If possible I would rather something closer to the following syntax...
context.Clubs.Search(club => new[]{ club.Name, club.County}, searchTerm);
// ... or, even better, something similar to this...
context.Clubs.Search(club => new { club.Name, club.County}, searchTerm);
This will return any golf club with 'Essex' in the Name or as the County.
public static IQueryable<TSource> Search<TSource>(this IQueryable<TSource> source, string searchTerm, params Expression<Func<TSource, string>>[] stringProperties)
{
if (String.IsNullOrEmpty(searchTerm))
{
return source;
}
// The lamda I would like to reproduce:
// source.Where(x => x.[property1].Contains(searchTerm)
// || x.[property2].Contains(searchTerm)
// || x.[property3].Contains(searchTerm)...)
//Create expression to represent x.[property1].Contains(searchTerm)
var searchTermExpression = Expression.Constant(searchTerm);
//Build parameters
var parameters = stringProperties.SelectMany(prop => prop.Parameters);
Expression orExpression = null;
//Build a contains expression for each property
foreach (var stringProperty in stringProperties)
{
var checkContainsExpression = Expression.Call(stringProperty.Body, typeof(string).GetMethod("Contains"), searchTermExpression);
if (orExpression == null)
{
orExpression = checkContainsExpression;
}
//Build or expression for each property
orExpression = Expression.OrElse(orExpression, checkContainsExpression);
}
var methodCallExpression = Expression.Call(typeof(Queryable),
"Where",
new Type[] { source.ElementType },
source.Expression,
Expression.Lambda<Func<TSource, bool>>(orExpression, parameters));
return source.Provider.CreateQuery<TSource>(methodCallExpression);
}
The error
If I change the number of parameters supplied to 1:
Expression.Lambda<Func<TSource, bool>>(orExpression, parameters.First()));
I get a new error:
UPDATE
I have written a post on the work discussed in this question. Check it out on GitHub too.

Here we go; you were pretty close - as I noted in comments, the key piece here is to use ExpressionVisitor to re-write the trees in terms of the single parameter you want to keep:
using System;
using System.Linq;
using System.Linq.Expressions;
static class Program
{
static void Main()
{
var data = new[] { new Foo { A = "x1", B = "y1", C = "y1" }, new Foo { A = "y2", B = "y2", C = "y2" },
new Foo { A = "y3", B = "y3", C = "x3" } }.AsQueryable();
var result = data.Search("x", x => x.A, x => x.B, x => x.C);
foreach (var row in result)
{
Console.WriteLine("{0}, {1}, {2}", row.A, row.B, row.C);
}
}
class Foo
{
public string A { get; set; }
public string B { get; set; }
public string C { get; set; }
}
public class SwapVisitor : ExpressionVisitor
{
private readonly Expression from, to;
public SwapVisitor(Expression from, Expression to)
{
this.from = from;
this.to = to;
}
public override Expression Visit(Expression node)
{
return node == from ? to : base.Visit(node);
}
public static Expression Swap(Expression body, Expression from, Expression to)
{
return new SwapVisitor(from, to).Visit(body);
}
}
public static IQueryable<TSource> Search<TSource>(this IQueryable<TSource> source, string searchTerm, params Expression<Func<TSource, string>>[] stringProperties)
{
if (String.IsNullOrEmpty(searchTerm))
{
return source;
}
if (stringProperties.Length == 0) return source.Where(x => false);
// The lamda I would like to reproduce:
// source.Where(x => x.[property1].Contains(searchTerm)
// || x.[property2].Contains(searchTerm)
// || x.[property3].Contains(searchTerm)...)
//Create expression to represent x.[property1].Contains(searchTerm)
var searchTermExpression = Expression.Constant(searchTerm);
var param = stringProperties[0].Parameters.Single();
Expression orExpression = null;
//Build a contains expression for each property
foreach (var stringProperty in stringProperties)
{
// re-write the property using the param we want to keep
var body = SwapVisitor.Swap(stringProperty.Body, stringProperty.Parameters.Single(), param);
var checkContainsExpression = Expression.Call(
body, typeof(string).GetMethod("Contains"), searchTermExpression);
if (orExpression == null)
{
orExpression = checkContainsExpression;
}
else
{ // compose
orExpression = Expression.OrElse(orExpression, checkContainsExpression);
}
}
var lambda = Expression.Lambda<Func<TSource, bool>>(orExpression, param);
return source.Where(lambda);
}
}

LIKE operator in LINQ

Is there any way to compare strings in a C# LINQ expression similar to SQL's LIKE operator?
Suppose I have a string list. On this list I want to search a string. In SQL, I could write:
SELECT * FROM DischargePort WHERE PortName LIKE '%BALTIMORE%'
Instead of the above, query want a linq syntax.
using System.Text.RegularExpressions;
…
var regex = new Regex(sDischargePort, RegexOptions.IgnoreCase);
var sPortCode = Database.DischargePorts
.Where(p => regex.IsMatch(p.PortName))
.Single().PortCode;
My above LINQ syntax does not work. What have I got wrong?

Typically you use String.StartsWith/EndsWith/Contains. For example:
var portCode = Database.DischargePorts
.Where(p => p.PortName.Contains("BALTIMORE"))
.Single()
.PortCode;
I don't know if there's a way of doing proper regular expressions via LINQ to SQL though. (Note that it really does depend on which provider you're using - it would be fine in LINQ to Objects; it's a matter of whether the provider can convert the call into its native query format, e.g. SQL.)
EDIT: As BitKFu says, Single should be used when you expect exactly one result - when it's an error for that not to be the case. Options of SingleOrDefault, FirstOrDefault or First should be used depending on exactly what's expected.

Regex? no. But for that query you can just use:
string filter = "BALTIMORE";
(blah) .Where(row => row.PortName.Contains(filter)) (blah)
If you really want SQL LIKE, you can use System.Data.Linq.SqlClient.SqlMethods.Like(...), which LINQ-to-SQL maps to LIKE in SQL Server.

Well... sometimes it may be uncomfortable to use Contains, StartsWith or EndsWith especially when searching value determine LIKE statment e.g. passed 'value%' require from developer to use StartsWith function in expression. So I decided to write extension for IQueryable objects.
Usage
// numbers: 11-000-00, 00-111-00, 00-000-11
var data1 = parts.Like(p => p.Number, "%11%");
// result: 11-000-00, 00-111-00, 00-000-11
var data2 = parts.Like(p => p.Number, "11%");
// result: 11-000-00
var data3 = parts.Like(p => p.Number, "%11");
// result: 00-000-11
Code
public static class LinqEx
{
private static readonly MethodInfo ContainsMethod = typeof(string).GetMethod("Contains");
private static readonly MethodInfo StartsWithMethod = typeof(string).GetMethod("StartsWith", new[] { typeof(string) });
private static readonly MethodInfo EndsWithMethod = typeof(string).GetMethod("EndsWith", new[] { typeof(string) });
public static Expression<Func<TSource, bool>> LikeExpression<TSource, TMember>(Expression<Func<TSource, TMember>> property, string value)
{
var param = Expression.Parameter(typeof(TSource), "t");
var propertyInfo = GetPropertyInfo(property);
var member = Expression.Property(param, propertyInfo.Name);
var startWith = value.StartsWith("%");
var endsWith = value.EndsWith("%");
if (startWith)
value = value.Remove(0, 1);
if (endsWith)
value = value.Remove(value.Length - 1, 1);
var constant = Expression.Constant(value);
Expression exp;
if (endsWith && startWith)
{
exp = Expression.Call(member, ContainsMethod, constant);
}
else if (startWith)
{
exp = Expression.Call(member, EndsWithMethod, constant);
}
else if (endsWith)
{
exp = Expression.Call(member, StartsWithMethod, constant);
}
else
{
exp = Expression.Equal(member, constant);
}
return Expression.Lambda<Func<TSource, bool>>(exp, param);
}
public static IQueryable<TSource> Like<TSource, TMember>(this IQueryable<TSource> source, Expression<Func<TSource, TMember>> parameter, string value)
{
return source.Where(LikeExpression(parameter, value));
}
private static PropertyInfo GetPropertyInfo(Expression expression)
{
var lambda = expression as LambdaExpression;
if (lambda == null)
throw new ArgumentNullException("expression");
MemberExpression memberExpr = null;
switch (lambda.Body.NodeType)
{
case ExpressionType.Convert:
memberExpr = ((UnaryExpression)lambda.Body).Operand as MemberExpression;
break;
case ExpressionType.MemberAccess:
memberExpr = lambda.Body as MemberExpression;
break;
}
if (memberExpr == null)
throw new InvalidOperationException("Specified expression is invalid. Unable to determine property info from expression.");
var output = memberExpr.Member as PropertyInfo;
if (output == null)
throw new InvalidOperationException("Specified expression is invalid. Unable to determine property info from expression.");
return output;
}
}

In native LINQ you may use combination of Contains/StartsWith/EndsWith or RegExp.
In LINQ2SQL use method SqlMethods.Like()
from i in db.myTable
where SqlMethods.Like(i.field, "tra%ata")
select i
add Assembly: System.Data.Linq (in System.Data.Linq.dll) to use this feature.

As Jon Skeet and Marc Gravell already mentioned, you can simple take a contains condition. But in case of your like query, it's very dangerous to take a Single() statement, because that implies that you only find 1 result. In case of more results, you'll receive a nice exception :)
So I would prefer using FirstOrDefault() instead of Single():
var first = Database.DischargePorts.FirstOrDefault(p => p.PortName.Contains("BALTIMORE"));
var portcode = first != null ? first.PortCode : string.Empty;

A simple as this
string[] users = new string[] {"Paul","Steve","Annick","Yannick"};
var result = from u in users where u.Contains("nn") select u;
Result -> Annick,Yannick

Ideally you should use StartWith or EndWith.
Here is an example:
DataContext dc = new DCGeneral();
List<Person> lstPerson= dc.GetTable<Person>().StartWith(c=> c.strNombre).ToList();
return lstPerson;

You can call the single method with a predicate:
var portCode = Database.DischargePorts
.Single(p => p.PortName.Contains("BALTIMORE"))
.PortCode;

You can also use the EF function tested in .net5
public async Task<IEnumerable<District>> SearchDistrict(string query, int stateId)
{
return await _dbContext
.Districts
.Include(s => s.State)
.Where(s => s.StateId == stateId && EF.Functions.Like(s.Name, "$%{query}%"))
.AsNoTracking()
.ToListAsync();
}

.Where(e => e.Value.StartsWith("BALTIMORE"))
This works like "LIKE" of SQL...

List<Categories> categoriess;
private void Buscar()
{
try
{
categoriess = Contexto.Categories.ToList();
categoriess = categoriess.Where(n => n.CategoryID >= Convert.ToInt32(txtCatID.Text) && n.CategoryID <= Convert.ToInt32(txtCatID1.Text) && (n.CategoryName.Contains(txtCatName.Text)) ).ToList();

Like Extension Linq / SQL
LikeExtension Class
Tested in .NET 5
public static class LikeExtension {
private static string ColumnDataBase<TEntity, TKey>(IModel model, Expression<Func<TEntity, TKey>> predicate) where TEntity : class {
ITable table = model
.GetRelationalModel()
.Tables
.First(f => f
.EntityTypeMappings
.First()
.EntityType == model
.FindEntityType(predicate
.Parameters
.First()
.Type
));
string column = (predicate.Body as MemberExpression).Member.Name;
string columnDataBase = table.Columns.First(f => f.PropertyMappings.Count(f2 => f2.Property.Name == column) > 0).Name;
return columnDataBase;
}
public static IQueryable<TEntity> Like<TEntity, TKey>(this DbContext context, Expression<Func<TEntity, TKey>> predicate, string text) where TEntity : class {
string columnDataBase = ColumnDataBase(context.Model, predicate);
return context.Set<TEntity>().FromSqlRaw(context.Set<TEntity>().ToQueryString() + " WHERE [" + columnDataBase + "] LIKE {0}", text);
}
public static async Task<IEnumerable<TEntity>> LikeAsync<TEntity, TKey>(this DbContext context, Expression<Func<TEntity, TKey>> predicate, string text, CancellationToken cancellationToken) where TEntity : class {
string columnDataBase = ColumnDataBase(context.Model, predicate);
return await context.Set<TEntity>().FromSqlRaw(context.Set<TEntity>().ToQueryString() + " WHERE [" + columnDataBase + "] LIKE {0}", text).ToListAsync(cancellationToken);
}
public static async Task<IEnumerable<TEntity>> LikeAsync<TEntity, TKey>(this IQueryable<TEntity> query, Expression<Func<TEntity, TKey>> predicate, string text, CancellationToken cancellationToken) where TEntity : class {
DbSet<TEntity> entities = query as DbSet<TEntity>;
string columnDataBase = ColumnDataBase(entities.EntityType.Model, predicate);
return await entities.FromSqlRaw(query.ToQueryString() + " WHERE [" + columnDataBase + "] LIKE {0}", text).ToListAsync(cancellationToken);
}
public static IQueryable<TEntity> Like<TEntity, TKey>(this IQueryable<TEntity> query, Expression<Func<TEntity, TKey>> predicate, string text) where TEntity : class {
DbSet<TEntity> entities = query as DbSet<TEntity>;
string columnDataBase = ColumnDataBase(entities.EntityType.Model, predicate);
return entities.FromSqlRaw(query.ToQueryString() + " WHERE [" + columnDataBase + "] LIKE {0}", text);
}
}
Repository
public async Task<IEnumerable<TEntity>> LikeAsync<TKey>(Expression<Func<TEntity, TKey>> predicate, string text, CancellationToken cancellationToken) {
return await context.LikeAsync(predicate, text, cancellationToken);
}
public IQueryable<TEntity> Like<TKey>(Expression<Func<TEntity, TKey>> predicate, string text) {
return context.Like(predicate, text);
}
Use
IQueryable<CountryEntity> result = countryRepository
.Like(k => k.Name, "%Bra[sz]il%") /*Use Sync*/
.Where(w => w.DateRegister < DateTime.Now) /*Example*/
.Take(10); /*Example*/
Or
IEnumerable<CountryEntity> result = await countryRepository
.LikeAsync(k => k.Name, "%Bra[sz]il%", cancellationToken); /*Use Async*/
Or
IQueryable<CountryEntity> result = context.Countries
.Like(k => k.Name, "%Bra[sz]il%")
.Where(w => w.Name != null); /*Example*/
Or
List<CountryEntity> result2 = await context.Countries
.Like(k => k.Name, "%Bra[sz]il%")
.Where(w => w.Name != null) /*Example*/
.ToListAsync(); /*Use Async*/
Or
IEnumerable<CountryEntity> result3 = await context.Countries
.Where(w => w.Name != null)
.LikeAsync(k => k.Name, "%Bra[sz]il%", cancellationToken); /*Use Async*/

public static class StringEx
{
public static bool Contains(this String str, string[] Arr, StringComparison comp)
{
if (Arr != null)
{
foreach (string s in Arr)
{
if (str.IndexOf(s, comp)>=0)
{ return true; }
}
}
return false;
}
public static bool Contains(this String str,string[] Arr)
{
if (Arr != null)
{
foreach (string s in Arr)
{
if (str.Contains(s))
{ return true; }
}
}
return false;
}
}
var portCode = Database.DischargePorts
.Single(p => p.PortName.Contains( new string[] {"BALTIMORE"}, StringComparison.CurrentCultureIgnoreCase) ))
.PortCode;

Just add to string object extention methods.
public static class StringEx
{
public static bool Contains(this String str, string[] Arr, StringComparison comp)
{
if (Arr != null)
{
foreach (string s in Arr)
{
if (str.IndexOf(s, comp)>=0)
{ return true; }
}
}
return false;
}
public static bool Contains(this String str,string[] Arr)
{
if (Arr != null)
{
foreach (string s in Arr)
{
if (str.Contains(s))
{ return true; }
}
}
return false;
}
}
usage:
use namespase that contains this class;
var sPortCode = Database.DischargePorts
.Where(p => p.PortName.Contains(new string [] {"BALTIMORE"}, StringComparison.CurrentCultureIgnoreCase) )
.Single().PortCode;

#adobrzyc had this great custom LIKE function - I just wanted to share the IEnumerable version of it.
public static class LinqEx
{
private static readonly MethodInfo ContainsMethod = typeof(string).GetMethod("Contains");
private static readonly MethodInfo StartsWithMethod = typeof(string).GetMethod("StartsWith", new[] { typeof(string) });
private static readonly MethodInfo EndsWithMethod = typeof(string).GetMethod("EndsWith", new[] { typeof(string) });
private static Func<TSource, bool> LikeExpression<TSource, TMember>(Expression<Func<TSource, TMember>> property, string value)
{
var param = Expression.Parameter(typeof(TSource), "t");
var propertyInfo = GetPropertyInfo(property);
var member = Expression.Property(param, propertyInfo.Name);
var startWith = value.StartsWith("%");
var endsWith = value.EndsWith("%");
if (startWith)
value = value.Remove(0, 1);
if (endsWith)
value = value.Remove(value.Length - 1, 1);
var constant = Expression.Constant(value);
Expression exp;
if (endsWith && startWith)
{
exp = Expression.Call(member, ContainsMethod, constant);
}
else if (startWith)
{
exp = Expression.Call(member, EndsWithMethod, constant);
}
else if (endsWith)
{
exp = Expression.Call(member, StartsWithMethod, constant);
}
else
{
exp = Expression.Equal(member, constant);
}
return Expression.Lambda<Func<TSource, bool>>(exp, param).Compile();
}
public static IEnumerable<TSource> Like<TSource, TMember>(this IEnumerable<TSource> source, Expression<Func<TSource, TMember>> parameter, string value)
{
return source.Where(LikeExpression(parameter, value));
}
private static PropertyInfo GetPropertyInfo(Expression expression)
{
var lambda = expression as LambdaExpression;
if (lambda == null)
throw new ArgumentNullException("expression");
MemberExpression memberExpr = null;
switch (lambda.Body.NodeType)
{
case ExpressionType.Convert:
memberExpr = ((UnaryExpression)lambda.Body).Operand as MemberExpression;
break;
case ExpressionType.MemberAccess:
memberExpr = lambda.Body as MemberExpression;
break;
}
if (memberExpr == null)
throw new InvalidOperationException("Specified expression is invalid. Unable to determine property info from expression.");
var output = memberExpr.Member as PropertyInfo;
if (output == null)
throw new InvalidOperationException("Specified expression is invalid. Unable to determine property info from expression.");
return output;
}
}

Great custom LIKE function by #adobrzycdon't work for me in LINQPad.
Here version which work in LINQPad (tested on LINQPad 5 and LINQPad 6)
Code
void Main()
{
var users = from au in ApplicationUsers
select au;
users.Like(u => u.UserName, "Ada Byron").Dump();
users.Like(u => u.UserName, "%yro%").Dump();
users.Like(u => u.UserName, "% Byron").Dump();
users.Like(u => u.UserName, "Ada %").Dump();
users.Like(u => u.UserName, "%yro%").Like(u => u.UserName, "Ada %").Dump();
// => SQL =>
// DECLARE #p0 NVarChar(1000) = 'Ada %'
// DECLARE #p1 NVarChar(1000) = '%yro%'
// SELECT [t0].[UserName], ...
// FROM [ApplicationUsers] AS [t0]
// WHERE ([t0].[UserName] LIKE #p0) AND ([t0].[UserName] LIKE #p1)
}
// based on LinqEx by adobrzyc (https://stackoverflow.com/a/35636138/1351740)
public static class LinqExFork
{
private static readonly MethodInfo ContainsMethod = typeof(string).GetMethod("Contains", new[] { typeof(string) });
private static readonly MethodInfo StartsWithMethod = typeof(string).GetMethod("StartsWith", new[] { typeof(string) });
private static readonly MethodInfo EndsWithMethod = typeof(string).GetMethod("EndsWith", new[] { typeof(string) });
public static Expression<Func<TSource, bool>> LikeExpression<TSource, TMember>(Expression<Func<TSource, TMember>> property, string value)
{
var param = Expression.Parameter(typeof(TSource), "t");
var memberInfo = GetMemberInfo(property);
MemberExpression member;
if (memberInfo is PropertyInfo)
member = Expression.Property(param, memberInfo.Name);
else if (memberInfo is FieldInfo)
member = Expression.Field(param, memberInfo.Name);
else
throw new InvalidOperationException("Unable to determine propery or field info from expression.");
var startWith = value.StartsWith("%");
if (startWith)
value = value.Remove(0, 1);
var endsWith = value.EndsWith("%");
if (endsWith)
value = value.Remove(value.Length - 1, 1);
var constant = Expression.Constant(value);
Expression exp;
if (endsWith && startWith)
exp = Expression.Call(member, ContainsMethod, constant);
else if (startWith)
exp = Expression.Call(member, EndsWithMethod, constant);
else if (endsWith)
exp = Expression.Call(member, StartsWithMethod, constant);
else
exp = Expression.Equal(member, constant);
return Expression.Lambda<Func<TSource, bool>>(exp, param);
}
public static IQueryable<TSource> Like<TSource, TMember>(this IQueryable<TSource> source, Expression<Func<TSource, TMember>> parameter, string value)
{
return source.Where(LikeExpression(parameter, value));
}
private static MemberInfo GetMemberInfo(Expression expression)
{
var lambda = expression as LambdaExpression;
if (lambda == null)
throw new ArgumentNullException("expression");
MemberExpression memberExpr = null;
switch (lambda.Body.NodeType)
{
case ExpressionType.Convert:
memberExpr = ((UnaryExpression)lambda.Body).Operand as MemberExpression;
break;
case ExpressionType.MemberAccess:
memberExpr = lambda.Body as MemberExpression;
break;
}
if (memberExpr == null)
throw new InvalidOperationException("Specified expression is invalid. Unable to determine member info from expression.");
var output = memberExpr.Member as MemberInfo;
if (output == null)
throw new InvalidOperationException("Specified expression is invalid. Unable to determine member info from expression.");
return output;
}
}

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