How to dynamically use the PropertyType reflection attribute to create a respective typed Function<>?

Question

I want to use type returned by PropertyType to create a typed function. I found this similiar http://stackoverflow.com/questions/914578/using-type-returned-by-type-gettype-in-c but this mentions how to create a list but does not mention how we can create a Func<>. Please help me out.

Pseudocode:

PropertyInfo inf = typeof(SomeClass).GetProperty("PropertyName");
Type T=inf.PropertyType;
Expression<Func<SomeClass,T>> le = GetPropertyOrFieldByName<SomeClass,T>("PropertyName");

static Expression<Func<TSource, TResult>> GetPropertyOrFieldByName<TSource,TResult>(string propertyOrFieldName)
{ 
ParameterExpression item = Expression.Parameter(typeof(TSource), "expr");MemberExpression prop = LambdaExpression.PropertyOrField(item, propertyOrFieldName);
var expr = Expression.Lambda<Func<TSource, TResult>>(prop, new ParameterExpression[] { item });
expr.Compile();
return expr;
}

Answer 1

Simply use MakeGenericType(new Type[] { SomeClass, T }).

EDIT More detail:

Type T2 = typeof(Excpression<>).MakeGenericType(typeof(Func<,>).MakeGenericType(new Type[] { SomeClass, T }));
... Activator.CreateInstance(T2);

,

Answer 2

You can create the expression tree, but you can't declare its type in the code. Where you've got this:

Expression<Func<SomeClass,T>> le = ...

T has to be known at compile time (or be a type parameter). Instead, your T is a variable whose value is only known at execution time.

Now, the next question is whether you really need that anyway - or in what way you need it. What are you trying to do with the expression? Could you just use the nongeneric Expression class instead as the variable type?

If you genuinely want an expression tree, you don't need to worry about MakeGenericType etc - use the Expression.Property method.

Answer 3

Type parameter = ...;
Type result = ...;
Type your_type = typeof(Func<,>).MakeGenericType(new Type[] {parameter, result});

Answer 4

If you want to call the GetPropertyOrFieldByName with the PropertyType, this should work:

PropertyInfo inf = typeof(SomeClass).GetProperty("PropertyName");
Type T = inf.PropertyType;

object le =
    typeof([TypeThatDeclaresMethod]).GetMethod("GetPropertyOrFieldByName")
    .MakeGenericMethod(typeof(SomeClass), T)
    .Invoke(null, new object[] { "PropertyName" });

Assuming that the GetPropertyOrFieldByName method is a public static method.