I am not an Expression Tree master by any stretch of the imagination, what I have is code that looks like this:
int external = 10;
using(var session = new Session())
{
session.Add(new Product { Name = "test1", Price = 20 });
session.Add(new Product {Name = "test", Price = 10});
var product = session.Products.Where(p => p.Price == external).FirstOrDefault();
Assert.Equal(10, product.Price);
}
Session implements all the IQueryProvider, IQueryable interfaces you'd come to expect from a LINQ Provider.
When I evaluate the expression tree, everything goes to plan until I read the ConstantExpression for "external", at which point, I am at a loss as to how to move forward because:
//constant is ConstantExpression for "external" on the right side of the "p.Price == external" expression above.
var t = constant.GetType(); //evaluates to class called "<>c__DisplayClass2" - with a member named "external" that has the value 10.
The question is basically.. How can I just access the value of the member "external" - is there a way to accomplish this without using reflection? or am I cooked? What am I missing?
Your expression captures the external local variable, that's why an anonymous type is implicitly created by the compiler to wrap the captured variable. The right-hand side part of the equality is not a ConstantExpression, it's actually a MemberExpression, whose Expression property is a ConstantExpression of type <>c__DisplayClass2
You can access the value of the property as follows :
MemberExpression memberExpr = /* right-hand side of the equality */ as MemberExpression;
ConstantExpression constantExpr = memberExpr.Expression as ConstantExpression;
PropertyInfo prop = memberExpr.Member as PropertyInfo;
object value = prop.GetValue(constantExpr.Value, null);
I prefer something like this. It is more general and will work for a number of cases.
var operation = (BinaryExpression)predicate.Body;
var righthandValue= Expression.Lambda(operation.Right).Compile().DynamicInvoke();
Related
I am trying to dynamically generate binary expressions using Expression.MakeBinary:
public static BinaryExpression MakeBinary(
ExpressionType binaryType,
Expression left,
Expression right
)
A simple example of that for Property "Bar" of class "Foo" would be:
var param = Expression.Parameter(typeof(Foo));
var left = MemberExpression.Property(param, "Bar");
var propertyType = typeof(Foo).GetProperty("Bar").PropertyType;
var right = Expression.Constant(Convert.ChangeType("newValue", propertyType ));
var expression = Expression.MakeBinary(ExpressionType.Equal, left, right);
My goal is to be able to do the same thing as above, but instead of comparing "Bar" property with "newValue", in my case "Bar" is a dictionary and I want to compare a specific entry of that dictionary with "newValue". What I want to achieve would look like this (but this code is not valid):
var param = Expression.Parameter(typeof(Foo));
var left = MemberExpression.Property(param, "Bar[\"entryName\"]");
I would suppose what I am trying to do is possible, but I can't seem to find the right syntax/class/method to use. Obviously the code above returns an error, since Bar["entryName"] is not a property. But how can I build the expression based on a specific entry in a dictionary for a given Type?
The high level application of this is to be able to apply rules, using field/operator/compareValue as parameters, but being able to indicate the "path" to that field (depending on the class, sometimes it would be a plain property, or as asked here, an entry in a dictionary, etc.).
Thank you
You can use Expression.MakeIndex():
var param = Expression.Parameter(typeof(Foo));
var bar = MemberExpression.Property(param, "Bar");
Type dictionaryType = typeof(Foo).GetProperty("Bar").PropertyType;
PropertyInfo indexerProp = dictionaryType.GetProperty("Item");
var dictKeyConstant = Expression.Constant("entryName");
var dictAccess = Expression.MakeIndex(bar, indexerProp, new[] {dictKeyConstant});
var propertyType = indexerProp.PropertyType;
var right = Expression.Constant(Convert.ChangeType("newValue", propertyType ));
var expression = Expression.MakeBinary(ExpressionType.Equal, dictAccess, right);
The property name of an indexer is always "Item". The dictKeyConstant should be the expression for your dictionary's key.
I have the following class, for which usage is not important. What is important is method SetCacheItemSelector which takes one parameter, a select expression that projects Account entity to AccountCacheDTO:
public class AccountRepositoryCache : RepositoryCache<Account, AccountCacheDTO>
{
public AccountRepositoryCache()
{
SetCacheItemSelector(x => new AccountCacheDTO
{
Id = x.Id,
Login = x.Login
});
}
}
So signature for this method is:
public void SetCacheItemSelector(Expression<Func<TEntity, TCacheItem>> selector)
In this case, TEntity is Account class, and TCacheItem is AccountCacheDTO class.
Is there a way to use reflection to build select expression dynamically for all the properties that are matching for both Account class and AccountCacheDTO class?
Goal is to have method that would look like this:
public Expression<Func<TEntity, TCacheItem>> BuildSelector<TEntity, TCacheItem>()
{
... // implementation with reflection goes here
}
EDIT:
Here is final implementation (pretty much the same as the accepted answer):
public static Expression<Func<TSource, TTarget>> BuildSelector<TSource, TTarget>()
{
Type targetType = typeof(TTarget);
Type sourceType = typeof(TSource);
ParameterExpression parameterExpression = Expression.Parameter(sourceType, "source");
List<MemberBinding> bindings = new List<MemberBinding>();
foreach (PropertyInfo sourceProperty in sourceType.GetProperties().Where(x => x.CanRead))
{
PropertyInfo targetProperty = targetType.GetProperty(sourceProperty.Name);
if (targetProperty != null && targetProperty.CanWrite && targetProperty.PropertyType.IsAssignableFrom(sourceProperty.PropertyType))
{
MemberExpression propertyExpression = Expression.Property(parameterExpression, sourceProperty);
bindings.Add(Expression.Bind(targetProperty, propertyExpression));
}
}
NewExpression newExpression = Expression.New(targetType);
Expression initializer = Expression.MemberInit(newExpression, bindings);
return Expression.Lambda<Func<TSource, TTarget>>(initializer, parameterExpression);
}
I didn't test it, but you should be able to do something like: This is just to convey a general idea and you should be able to tweak it for your requirements.
public Expression<Func<TEntity, TCacheItem>> BuildSelector<TEntity, TCacheItem>(TEntity entity)
{
List<MemberBinding> memberBindings = new List<MemberBinding>();
MemberInitExpression body = null;
foreach (var entityPropertyInfo in typeof(TEntity).GetProperties())
{
foreach (var cachePropertyInfo in typeof(TCacheItem).GetProperties())
{
if (entityPropertyInfo.PropertyType == cachePropertyInfo.PropertyType && entityPropertyInfo.Name == cachePropertyInfo.Name)
{
var fieldExpressoin = Expression.Field(Expression.Constant(entity), entityPropertyInfo.Name);
memberBindings.Add(Expression.Bind(cachePropertyInfo, fieldExpressoin));
}
}
}
var parameterExpression = Expression.Parameter(typeof(TEntity), "x");
var newExpr = Expression.New(typeof(TCacheItem));
body = Expression.MemberInit(newExpr, memberBindings);
return Expression.Lambda<Func<TEntity, TCacheItem>>(body, parameterExpression);
}
Of course, the #Aravol's answer can make sense, but it is a little different which required in OP. Here is the solution which is more suitable to OP requirement.
public Expression<Func<TEntity, TCacheItem>> BuildSelector<TEntity, TCacheItem>()
{
Type type = typeof(TEntity);
Type typeDto = typeof(TCacheItem);
var ctor = Expression.New(typeDto);
ParameterExpression parameter = Expression.Parameter(type, "p");
var propertiesDto = typeDto.GetProperties(BindingFlags.Public | BindingFlags.Instance);
var memberAssignments = propertiesDto.Select(p =>
{
PropertyInfo propertyInfo = type.GetProperty(p.Name, BindingFlags.Public | BindingFlags.Instance);
MemberExpression memberExpression = Expression.Property(parameter, propertyInfo);
return Expression.Bind(p, memberExpression);
});
var memberInit = Expression.MemberInit(ctor, memberAssignments);
return Expression.Lambda<Func<TEntity, TCacheItem>>(memberInit, parameter);
}
Your best bet is to get very comfortable with the System.Linq.Expressions namespace, which contains all of the methods you'll need to dynamically metacode your method calls and compile them into delegates. See especially Expression.Call and Lambda.Compile methods. Note that using Lambda.Compile, you can also have a true, compiled Delegate, instead of an expression tree (Expression) wrapping the call to your desired method. (NOTE: You can also forgo the Compile step if you really want that expression tree for later)
As for building your set, that's Assembly scanning, and is going to be a matter of iterating over all classes in your Assembly. I highly recommend you utilize, at the very least, a custom Attribute on your assembly or future assemblies to mark them for this scan, lest this process end up much more costly. At the most, you should consider using a custom Attribute to mark which properties you want scanned for this expression build.
the actual code to this tends to start with
AppDomain.CurrentDomain // Necessary to get all available Assemblies
.GetAssemblies() // Gets all the assemblies currently loaded in memory that this code can work with
.AsParallel() // Highly recommended to make the attribute-checking steps run asynchronously
// Also gives you a handy .ForAll Method at the end
// TODO: .Where Assembly contains Attribute
.SelectMany(assembly => assembly.GetTypes())
// TODO: .Where Type contains Attribute
.SelectMany(type => type.GetProperties)
// TODO: Make sure Property has the right data...
.Select(CompileFromProperty)
Where CompileFromProperty is a method taking PropertyInfo and returning the desired Expression.
Look into ToList() and ToDictionary after that, as you may need to break out of the parallelization once you start pushing values to your cache
Addendum: you also have .MakeGenericType on the Type class, which will allow you to specify Generic parameters from other Type variables, which will prove invaluable when building the Expressions. Don't forget about Contravariance when you define the generic types!
I have a variable called sortColumn, which contains the text of a column that I want to sort a query result by. I also have a generic repository which takes as a parameter an Expression that contains the field I want to sort by. I can't seem to get from the string property name to an Expression.
So the generic repository that I have contains the following method
public IEnumerable<TEntity> Get<TOrderBy>(Expression<Func<TEntity, bool>> criteria,
Expression<Func<TEntity, TOrderBy>> orderBy, int pageIndex,
int pageSize,
bool isAssendingOrder = true,
EnumDeletePolicy deletePolicy = EnumDeletePolicy.ExcludeDeleted)
Notice the second parameter to this Get is Expression-Func-TEntity, TOrderBy. As I mentioned I have a variable called sortColumn, which contains the string for a property on my TEntity object I need to convert this string into an Expression that I can pass to the Get method.
Here is what I have right now.
var parameter = Expression.Parameter(typeof(IContract));
var memberExpression = Expression.Property(parameter, data.SortColumn);
var lambdaExpression = Expression.Lambda(memberExpression, parameter);
Which creates an object of type LambdaExpression. The actual type of this LambdaExpression is an Expression-Func-IContract, string (or whatever the type sortColumn of the property is). If I call the Get method and pass in this LambdaExpression and explicitly cast it to the Expression type then it will work fine. The problem is I don't know what the Expression type is, it could be a string, int, int?, etc. It all depends on the type of the property that is specific in the sortColumn property.
Can you help me make this last jump to the right Expression type?
Edit based on Marc's suggestions:
I nearly have this working, actually based specifically on the question it is working, but I have 1 remaining problem.
The IContract which is the Entity Type that I'm querying against actually inherits from IRelationship. If I specify a field from the IContract interface then the code above works. If I specify a field from the IRelationship interface then the following line fails.
var memberExpression = Expression.Property(parameter, data.SortColumn);
If I try something like below so that I'm grabbing the MemberExpression from the IRelationship, but building the Lambda based on IContract I get an error from the repository.
var parameter = Expression.Parameter(typeof(IRelationship));
var memberExpression = Expression.Property(parameter, data.SortColumn);
var orderBy = Expression.Lambda(memberExpression, Expression.Parameter(typeof(IContract)));
The error that I get is "The parameter '' was not bound in the specified LINQ to Entities query expression."
The final expression to get it working was this
var parameter = Expression.Parameter(typeof(IContract));
var memberExpression = Expression.Property(parameter, typeof(IRelationship), data.SortColumn);
var orderBy = Expression.Lambda(memberExpression, parameter);
So I needed to specify the middle parameter to the memberExpression line, to say look in the inherited Relationship interface for the property
You kinda need to use the correct generic overload - which used to mean you had to use MakeGenericMethod; however, you can also use dynamic to avoid the need to use MakeGenericMethod here, for example (in this case via Where, but the important point is how it works):
IQueryable<Foo> source = new[] { new Foo { Bar = 123 } }.AsQueryable();
Expression<Func<Foo,bool>> typed = x=>x.Bar == 123;
LambdaExpression untyped = typed;
IQueryable<Foo> filtered = Queryable.Where(source, (dynamic)untyped);
Note: you can't use extension methods here - hence why you need to use Queryable.*.
For an OrderBy example using your code:
var parameter = Expression.Parameter(typeof(Foo));
var memberExpression = Expression.Property(parameter, "Bar");
var lambdaExpression = Expression.Lambda(memberExpression, parameter);
LambdaExpression untyped = lambdaExpression;
IQueryable<Foo> sorted = Queryable.OrderBy(source, (dynamic)untyped);
var all = sorted.ToArray();
Re the edit:
var parameter = Expression.Parameter(typeof(IRelationship));
var memberExpression = Expression.Property(
Expression.Convert(parameter, typeof(IContract)), data.SortColumn);
var orderBy = Expression.Lambda(memberExpression, parameter);
I have an expression tree function from a previous SO question. It basically allows the conversion of a data row into a specific class.
This code works fine, unless you're dealing with data types that can be bigger or smaller (eg. Int32/Int64).
The code throws an invalid cast exception when going from an Int64 to an Int32 when the value would fit in an Int32 (eg. numbers in the 3000).
Should I?
Attempt to fix this in the code? (If so, any pointers?)
Leave the code as it is.
private Func<SqlDataReader, T> getExpressionDelegate<T>()
{
// hang on to row[string] property
var indexerProperty = typeof(SqlDataReader).GetProperty("Item", new[] { typeof(string) });
// list of statements in our dynamic method
var statements = new List<Expression>();
// store instance for setting of properties
ParameterExpression instanceParameter = Expression.Variable(typeof(T));
ParameterExpression sqlDataReaderParameter = Expression.Parameter(typeof(SqlDataReader));
// create and assign new T to variable: var instance = new T();
BinaryExpression createInstance = Expression.Assign(instanceParameter, Expression.New(typeof(T)));
statements.Add(createInstance);
foreach (var property in typeof(T).GetProperties())
{
// instance.MyProperty
MemberExpression getProperty = Expression.Property(instanceParameter, property);
// row[property] -- NOTE: this assumes column names are the same as PropertyInfo names on T
IndexExpression readValue = Expression.MakeIndex(sqlDataReaderParameter, indexerProperty, new[] { Expression.Constant(property.Name) });
// instance.MyProperty = row[property]
BinaryExpression assignProperty = Expression.Assign(getProperty, Expression.Convert(readValue, property.PropertyType));
statements.Add(assignProperty);
}
var returnStatement = instanceParameter;
statements.Add(returnStatement);
var body = Expression.Block(instanceParameter.Type, new[] { instanceParameter }, statements.ToArray());
var lambda = Expression.Lambda<Func<SqlDataReader, T>>(body, sqlDataReaderParameter);
// cache me!
return lambda.Compile();
}
Update:
I have now given up and decided it is not worth it. From the comments below, I got as far as:
if (readValue.Type != property.PropertyType)
{
BinaryExpression assignProperty = Expression.Assign(getProperty, Expression.Convert(Expression.Call(property.PropertyType, "Parse", null, new Expression[] { Expression.ConvertChecked(readValue, typeof(string)) }), property.PropertyType));
statements.Add(assignProperty);
}
else
{
// instance.MyProperty = row[property]
BinaryExpression assignProperty = Expression.Assign(getProperty, Expression.Convert(readValue, property.PropertyType));
statements.Add(assignProperty);
}
I don't think I was too far off, feel free to finish it and post the answer if you figure it out :)
You could try to fix it by "convert checked" before assigning i.e. using Expression.ConvertChecked on the value instead of Expression.Convert .
Couldn't try it right now but this should take care of the case you describe...
EDIT - as per comment this could be a boxing issue:
In this case you could try using Expression.TypeAs or Expression.Unbox for the conversion or use Expression.Call for calling a method to do the conversion... an example for using Call can be found at http://msdn.microsoft.com/en-us/library/bb349020.aspx
What you're trying to build is actually much more complicated if you want to support 100% of the primitives in .NET and SQL.
If you don't care about some of the edge cases (nullable types, enums, byte arrays, etc), two tips to get you 90% there:
Don't use the indexer on IDataRecord, it returns an object and the boxing/unboxing will kill performance. Instead, notice that IDataRecord has Get[typeName] methods on it. These exist for all .NET primitive types (note: it's GetFloat, not GetSingle, huge annoyance).
You can use IDataRecord.GetFieldType to figure out which Get method you need to call for a given column. Once you have that, you can use Expression.Convert to coerce the DB column type to the target property's type (if they're different). This will fail for some of the edge cases I listed above, for those you need custom logic.
I am trying to dynamically build an expression tree in C#, which is compiled and used as the predicate for LINQ-to-SQL Where() call. The problem is that I am trying to compare an Enum (with int as its underlying type) directly against an Int, but this is failing with the error "The member MyEnumType has no supported translation to SQL".
Code:
ParameterExpression param = Expression.Parameter(typeof(MyClass), "obj"); //input parameter - this exposes a property of the Enum type
MemberExpression enumMember = Expression.Property(param, "MyEnumProperty"); //used to get the Enum typed property from the parameter
//MISSING STEP TO CAST THE ENUM OF THE MEMBER EXPRESSION TO AN INT?
BinaryExpression binaryExpr = Expression.Equal(enumMember, Expression.Constant(1));
LambdaExpression<Func<MyClass, bool>> whereClause = Expression.Lambda(binaryExpr, param);
//when whereClause is used to filter LINQ-to-SQL results, the error is thrown
I'm fairly new to expression trees and I can't figure this out. I have tried using
Expression.Convert(enumMember, typeof(int))
as the first part of the BinaryExpression but this doesn't fix it.
Any help is much appreciated.
Simply, you shouldn't have to, as long as you've told LINQ-to-SQL about the enum (rather than mapping it as an int and having a separate property in C# that does the translation). For example, the following works fine:
var param = Expression.Parameter(typeof(DomainObject));
var body = Expression.Equal(Expression.Property(param, "SomeProperty"),
Expression.Constant(YourEnumType.SomeEnum));
var predicate = Expression.Lambda<Func<DomainObject, bool>>(body, param);
var count = db.Table.Where(predicate).Count();
The main point is that my SomeProperty property is mapped in the dbml to the enum. Simply overtype the type name with the enum type (including namespace).
Likewise, you shouldn't be giving it a 1, but rather the typed enum; for example:
Expression.Constant(Enum.ToObject(typeof(YourEnumType), 1))
(if all you know is 1)
Thanks To Marc Gravell. (Expression Guru !) See Correct answer.
I made a change to an Expression Routine to cater for this scenario.
Normal properties or Enums. Incase someone finds this helpful
public static Expression<Func<TPoco, bool>> GetEqualsPredicate<TPoco>(string propertyName,
object value)
Type fieldType )
{
var parameterExp = Expression.Parameter(typeof(TPoco), #"t"); //(tpoco t)
var propertyExp = Expression.Property(parameterExp, propertyName);// (tpoco t) => t.Propertyname
var someValue = fieldType.IsEnum // get and eXpressionConstant. Careful Enums must be reduced
? Expression.Constant(Enum.ToObject(fieldType, value)) // Marc Gravell fix
: Expression.Constant(value, fieldType);
var equalsExp = Expression.Equal(propertyExp, someValue); // yes this could 1 unreadble state if embedding someValue determination
return Expression.Lambda<Func<TPoco, bool>>(equalsExp, parameterExp);
}
look my friend first of all you have to modify your enum to be like that:
public enum myenum : int
{
item1 = 0,
item2 = 1
}
after that you can convert between int and that eunm by that way:
int x = (int) myenum.item1;
just you must convert enum to object
you can use this :
Expression.Constant(Enum.ToObject(enumMember.Type, enumMember .value))
Try
(int) enumMember