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Linq extending Expressions

I'm trying to write a generic wildcard Search for the ServiceStack.OrmLite.SqlExpressionVisitor that has the following signature:
public static SqlExpressionVisitor<T> WhereWildcardSearch<T> (this SqlExpressionVisitor<T> ev, Expression<Func<T,string>> field, string search)
where ev is the rest of the filter, field is the getter for the field to search by and search is the entered term.
Normally (non-generic) I would write the following:
if(search.StartsWith('*') && search.EndsWith('*'))
ev = ev.Where(x => x.foo.Contains(search.Trim('*')));
and of course also variants for x.foo.StartsWith or EndsWith.
Now I am searching for something like (pseudocode:)
ev = ev.Where(x => field(x).Contains(search.Trim('*')));
Of course I can't compile and call the expression directly, as this should be translated to Sql using Linq2Sql.
This is my code so far:
var getFieldExpression = Expression.Invoke (field, Expression.Parameter (typeof (T), "getFieldParam"));
var searchConstant = Expression.Constant (search.Trim('*'));
var inExp = Expression.Call (getFieldExpression, typeof(String).GetMethod("Contains"), searchConstant);
var param = Expression.Parameter (typeof (T), "object");
var exp = Expression.Lambda<Func<T, bool>> (inExp, param);
ev = ev.Where (exp);
Please don't tell me that I should directly write SQL with $"LIKE %search%" or something - I know that there are other ways, but solving this would help my understanding of Linq and Expressions in general and it bugs me when I can't solve it.

Here is how it can be done (I think it will be clear for you without much additional explanations what you did wrong, but if not - feel free to request a clarification):
// extract property name from passed expression
var propertyName = ((MemberExpression)field.Body).Member.Name;
var param = Expression.Parameter(typeof(T), "object");
var searchConstant = Expression.Constant(search.Trim('*'));
var contains = typeof(String).GetMethod("Contains");
// object.FieldName.Contains(searchConstant)
var inExp = Expression.Call(Expression.PropertyOrField(param, propertyName), contains, searchConstant);
// object => object.FieldName.Contains(searchConstant)
var exp = Expression.Lambda<Func<T, bool>>(inExp, param);
In response to comment. You have two expression trees: one is being passed to you and another one which you are building (exp). In this simple case they both use the same number of parameters and those parameters are of the same type (T). In this case you can reuse parameter from field expression tree, like this:
// use the same parameter
var param = field.Parameters[0];
var searchConstant = Expression.Constant(search.Trim('*'));
var contains = typeof(String).GetMethod("Contains");
// note field.Body here. Your `field` expression is "parameter => parameter.Something"
// but we need just "parameter.Something" expression here
var inExp = Expression.Call(field.Body, contains, searchConstant);
// pass the same parameter to new tree
var exp = Expression.Lambda<Func<T, bool>>(inExp, param);
In more complicated cases you might need to use ExpressionVisitor to replace parameters in one expression tree to reference to parameters from another (final) expression tree.

Rewrite Lambda expression with System.Linq.Expressions APIs

I am stuck on this for hours. All I want to do is build an Expression tree by rewriting this following expression using Expression class APIs:
var Expression<Func<T, bool>> expr = x => x.SomeProperty == value;
What I got so far are:
{
var param = Expression.Parameter(typeof(T), "x");
var lhs = Expression.Property(param, "SomeProperty");
var rhs = Expression.Constant(value, value.GetType());
return Expression.Call(typeof(object).GetMethod("Equals", BindingFlags.Static | BindingFlags.Public), lhs, rhs);
}
This works fine if T is a primitive type or enumeration. But I got an exception if T is a reference type, a class etc.
Exception Message:
Unable to create a constant value of type 'TypeName'. Only primitive
types or enumeration types are supported in this context.
Thanks in advance.

You don't need to specify the type explicitly, in this case, as long as the value is not null (which I'm assuming it isn't, as you're calling GetType() on it).
This should do it.
var param = Expression.Parameter(typeof(T), "x");
var property = Expression.Property(param, "SomeProperty");
var compareValue = Expression.Constant(value);
var equals = Expression.Equal(property, compareValue);
return Expression.Lambda<Func<T, bool>>(equals, param);

The expression generated was passed to a Linq Where call. Like this.
Context.Sources.Where(criteria.BuildQuery());
The exception was thrown when the expression is being evaluated/translated.
If I compile the expression and then pass a delegate to the Where call, everything works as expected.
Context.Sources.Where(criteria.BuildQuery().Compile());
I am not sure what difference it makes, if someone knows why please enlighten us.

Build LambdaExpression with Func<,> where returnType is IQueryable<T>

I am trying to create a generic way of getting an EntityFramework object based on its own id without passing in a lambda expression as parameter to the method GetById(). For the code below the entity T is of type Message, is known to the class where GetById() is implemented and has a property MessageId along with several other properties. The MessageId name has been hard-coded in the example below as this is still experimental - extracting the id property name from T is quite easy to fix later.
I have been struggling to find a way to construct a simple LambdaExpression which has IQueryable<T> as parameter type and hope that someone would have a clue on how this could be done. The reason why I want IQueryable<T> is because my underlying channel factory provider requires this for more complex queries.
The line with var exp = Expression.Lambda<...> in the code below shows the expression function type definition which I want to end up with, but the line gives the exception:
Expression of type System.Boolean cannot be used for return type IQueryable
That's because the body has the Boolean type while my expression parameter queryParamtRet is of type IQueryable<Message>. Further, if I change the body type to be an IQueryable<Message>, I'm not able to find the property MessageId since the type is no longer type T as Message but type IQueryable<T>.
public T GetById(int id)
{
var queryParamLeft = Expression
.Parameter(typeof(System.Data.Entity.DbSet<T>), "o");
var queryParamRet = Expression
.Parameter(typeof(IQueryable<T>), "o");
var entityFrameworkType = Expression
.Parameter(typeof(T), "o");
var queryProperty = Expression
.PropertyOrField(entityFrameworkType, "MessageId");
var body = Expression
.Equal(queryProperty, Expression.Constant(id));
var exp = Expression
.Lambda<Func<System.Data.Entity.DbSet<T>, IQueryable<T>>>(
body,
queryParamRet);
var returnXml = DoWithChannel(channel
=> channel.Load(serializer.Serialize(exp)));
}

TLDR: Write out code that you want to create an expression for, then deliberately create the expression, starting with any inner expressions before combining them into the outer expression.
If you write your intended code as a function, it would look something like this
public static IQueryable<T> FilterADbSet(DbSet<T> dbSet)
{
return Queryable.Where<T>(dbSet, o => o.MessageId == 34);
}
It has one input parameter of type DbSet<T>, an output of type IQueryable<T> and it calls Queryable.Where<T> with parameters of the dbSet variable and an expression.
Working from the outside in, you first need to build the expression to pass to the where clause. You have already done that in your code.
Next you need to create a lambda expression for the where clause.
var whereClause = Expression.Equal(queryProperty, Expression.Constant(id));
var whereClauseLambda = Expression.Lambda<Func<T, bool>>(whereClause, entityFrameworkType);
Next, as the comments indicate, you need to use Expression.Call to create a body.
My end result with making your code work is below.
static Expression<Func<IQueryable<T>, IQueryable<T>>> WhereMethodExpression = v => v.Where(z => true);
static MethodInfo WhereMethod = ((MethodCallExpression)WhereMethodExpression.Body).Method;
public T GetById(int id)
{
var queryParamLeft = Expression
.Parameter(typeof(System.Data.Entity.DbSet<T>), "dbSet");
var entityFrameworkType = Expression
.Parameter(typeof(T), "entity");
var queryProperty = Expression
.PropertyOrField(entityFrameworkType, "MessageId");
var whereClause = Expression
.Equal(queryProperty, Expression.Constant(id));
var whereClauseLambda = Expression.Lambda<Func<T, bool>>(whereClause, entityFrameworkType);
var body = Expression.Call(
WhereMethod,
queryParamLeft,
whereClauseLambda
);
var exp = Expression
.Lambda<Func<System.Data.Entity.DbSet<T>, IQueryable<T>>>(
body,
queryParamLeft);
var returnXml = DoWithChannel(channel
=> channel.Load(serializer.Serialize(exp)));
}
I used an expression to fetch the MethodInfo object of Queryable.Where<T>
Your body expression needed queryParamLeft passed in. queryParamRet is not needed

LINQ: Why does Expression.Call() reference the Db when CreateQuery() does as well?

So I'm trying to optimize a query that does a text search on 1000s of rows and displays 100s of them. To do so, I'm trying to understand some code from Microsoft's 101 Sample Queries (found here).
Here is the code I wish to understand:
[Category("Advanced")]
[Title("Dynamic query - Where")]
[Description("This sample builds a query dynamically to filter for Customers in London.")]
public void LinqToSqlAdvanced02()
{
IQueryable<Customer> custs = db.Customers;
ParameterExpression param = Expression.Parameter(typeof(Customer), "c");
Expression right = Expression.Constant("London");
Expression left = Expression.Property(param, typeof(Customer).GetProperty("City"));
Expression filter = Expression.Equal(left, right);
Expression pred = Expression.Lambda(filter, param);
Expression expr = Expression.Call(typeof(Queryable), "Where", new Type[] { typeof(Customer) }, Expression.Constant(custs), pred);
IQueryable<Customer> query = db.Customers.AsQueryable().Provider.CreateQuery<Customer>(expr);
ObjectDumper.Write(query);
}
So...I realize this is esoteric, but why does Expression.Call() need the DbSet reference it's passed to CreateQuery as <T>?

I believe what you're asking is why the third parameter (array of types) is needed in Expression.Call because if you did it through code, you'd only have to do this:
custs.Where(pred);
The reason why the code works without the generic parameter is because of implicit typing, where the compiler translates that automatically to:
custs.Where<Customer>(pred);
After it translates it, the actual bytecode contains the call with the generics specified. When you build the Expression, you don't have all the niceties like implicit typing, so you have to specify exactly what gets called.

Create fully dynamic where clause with expression tree and execute on IQueryable

At point (3) in my code I have defined a query called query1 in which I defined a .Where lambda expression. This query is in some way dynamic but still contains static elements, it always refers to the Type Employee and its (int) property ClientID.
Now I very much like to make the refering to the type and its property dynamic, based on the method parameters which by example are shown below point (1).
What I tried to so far is making the static part of the query defined under point (3) fully dynamic by replacing it with a more elaborate expression tree as written down in (4), (5) & (6). But when I try to add everything together it says I call .Where with wrong parameters. I don't know how to call .Where with the right parameters in order to create a fully dynamic select.
Does someone know to solve this problem? I have spent a day searching and haven't found a solution so far.
dsMain domainService = new dsMain();
//(1)i want to rewrite the following four variables to method-parameters
Type entityType = typeof(Employee);
String targetProperty = "ClientID";
Type entityProperty = typeof(Employee).GetProperty(targetProperty).PropertyType;
int idToDelete = 5;
//(2)create expression-function: idToDelete == entityType.targetProperty (in this case: Employee.ClientID)
ParameterExpression numParam = Expression.Parameter(entityProperty, targetProperty.Substring(0, 3));
ConstantExpression equalTarget = Expression.Constant(idToDelete, idToDelete.GetType());
BinaryExpression intEqualsID = Expression.Equal(numParam, equalTarget);
Expression<Func<int, bool>> lambda1 =
Expression.Lambda<Func<int, bool>>(
intEqualsID,
new ParameterExpression[] { numParam });
//(3)I want to create query1 fully dynamic, so defining Employee or an other type and its property at run time
WhereClause = lambda1.Compile();
IQueryable<Employee> employees = domainService.GetEmployees();
var query1 = employees.Where<Employee>(C => WhereClause.Invoke(C.ClientID)).Expression;
//(4)create the operand body {value(ASP.test_aspx).WhereClause.Invoke(E.ClientID)}
var operandbodyMethod = WhereClause.GetType().GetMethod("Invoke");
var operandbodyType = typeof(System.Boolean);
var operandbodyArgs1Expression = Expression.Parameter(entityType, entityType.Name.Substring(0, 1));
var operandbodyArgs1 = Expression.MakeMemberAccess(operandbodyArgs1Expression, entityType.GetMember(targetProperty)[0]);
var operandBodyObjectExp = Expression.Constant(this, this.GetType());
var operandbodyObject = Expression.MakeMemberAccess(operandBodyObjectExp, this.GetType().GetMember("WhereClause")[0]);
//(5)create the operand {E => value(ASP.test_aspx).WhereClause.Invoke(E.ClientID)}
var operandbody = Expression.Call(operandbodyObject, operandbodyMethod, operandbodyArgs1);
var operandParameter = Expression.Parameter(entityType, entityType.Name.Substring(0, 1));
var operandType = typeof(Func<,>).MakeGenericType(entityType, typeof(System.Boolean));
//(6)
var operand = Expression.Lambda(operandType, operandbody, new ParameterExpression[] { operandParameter });
var expressionType = typeof(Expression<>).MakeGenericType(operandType);
var completeWhereExpression = Expression.MakeUnary(ExpressionType.Quote, operand, expressionType);
//(7)the line below does not work
var query2 = employees.Where<Employee>(completeWhereExpression).Expression;
Thank you very much for reading my question!
If you have questions about my question, please ask them:)

This is quite hard to look at in isolation, but the first thing that occurs is that Compile looks out of place for IQueryable - that will rarely work (LINQ-to-Objects being the exception).
An equivalent to WhereClause.Invoke(C.ClientID) is to use Expression.Invoke to call a sub-expression, but even this is flakey: LINQ-to-SQL will support it, EF (in 3.5 at least) doesn't (maybe "didn't"; I haven't re-checked in 4.0). Ultimately, it would be more robust to create lambda1 as an Expression<Func<Employee,bool>> if possible:
ParameterExpression empParam = Expression.Parameter(typeof(Employee),"emp");
ConstantExpression equalTarget = Expression.Constant(idToDelete, idToDelete.GetType());
BinaryExpression intEqualsID = Expression.Equal(
Expression.PropertyOrField(empParam, targetProperty), equalTarget);
Expression<Func<Exmployee, bool>> lambda1 =
Expression.Lambda<Func<int, bool>>(
intEqualsID,
empParam);
Then pass this to Where:
var query1 = employees.Where(lambda1);