How can I build an expression that compiles to a Func<T>?

Question

Suppose I have a static method that returns T such as:

T myT = MyClass.Create(type); // type is System.Type

I then want to be able to build and compile an expression so that I can have a Func<T> but I cannot figure out how to do it.

I can build it for a Constant doing:

Func<T> result = Expression.Lambda<Func<T>>(Expression.Constant(string.Empty)).Compile()

but for MyClass.Create(type) I am stuck.

Func<T> result = ....?

Answer 1

Thanks to the tip from usr I managed to do it using Expression.Call so given:

public static class MyClass
{
    public static string GetTime() 
    {
        return DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss,fff");
    }

    public static string GetName(Type type)
    {
        return type.Name;
    }
}

Then:

// Calls static method GetTime with no args
var mCallGetTime = Expression.Call(typeof(MyClass), "GetTime", null);
Func<string> resultNoArg = Expression.Lambda<Func<string>>(mCallGetTime).Compile();

// The input param for GetName which is of type Type
var paramExp = Expression.Parameter(typeof(Type));

// Calls static method GetName passing in the param
var mCallGetName = Expression.Call(typeof(MyClass), "GetName", null, paramExp);
Func<Type, string> resultWithArg = Expression.Lambda<Func<Type, string>>(mCallGetName, paramExp).Compile();

// You can then call them like so... Print() just prints the string
resultNoArg().Print();
resultWithArg(typeof(string)).Print();
resultWithArg(typeof(int)).Print();

Alternatively, Instead of:

var mCallGetTime = Expression.Call(typeof(MyClass), "GetTime", null);

We could compose the Expression.Call using:

// Get the method info for GetTime (note the Type[0] signifying no args taken by GetTime);
var methodInfo = typeof(MyClass).GetMethod("GetTime", new Type[0]);
var mCallGetTime = Expression.Call(methodInfo);

Likewise for GetName:

// Get the method info for GetName (note the new[] {typeof(Type)} signifying the arg taken by GetName
var getNameMethodInfo = typeof(MyClass).GetMethod("GetName", new[] { typeof(Type)});
var mCallGetName = Expression.Call(getNameMethodInfo, paramExp);