How to use Reflection to Invoke an Overloaded Method in .NET

Question

Is there a way to Invoke an overloaded method using reflection in .NET (2.0). I have an application that dynamically instantiates classes that have been derived from a common base class. For compatibility purposes, this base class contains 2 methods of the same name, one with parameters, and one without. I need to call the parameterless method via the Invoke method. Right now, all I get is an error telling me that I'm trying to call an ambiguous method.

Yes, I could just cast the object as an instance of my base class and call the method I need. Eventually that will happen, but right now, internal complications will not allow it.

Any help would be great! Thanks.

score 134 · Accepted Answer · edited Jul 21 '17 at 23:55

134

You have to specify which method you want:

class SomeType 
{
    void Foo(int size, string bar) { }
    void Foo() { }
}

SomeType obj = new SomeType();
// call with int and string arguments
obj.GetType()
    .GetMethod("Foo", new Type[] { typeof(int), typeof(string) })
    .Invoke(obj, new object[] { 42, "Hello" });
// call without arguments
obj.GetType()
    .GetMethod("Foo", new Type[0])
    .Invoke(obj, new object[0]);

edited Jul 21 '17 at 23:55

JG in SD

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answered Oct 21 '08 at 21:10

Hallgrim

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You can also do Type.EmptyTypes – yoel halb Apr 11 '14 at 15:34
2

need } after "typeof(int), typeof(string)" to compile :) – Omid.Hanjani May 31 '15 at 14:30
2

What if one of the args is generic? – Steven Turner Mar 29 '19 at 03:28
When you have an object instance you know what types it has been instantiated with. – Hallgrim May 01 '19 at 14:06
And if you need to specify an out parameter, do not mess with modifiers, that does not work (for me). You need to pass the type object with its IsByRef property set to true. IsByRef is readonly though, you need to use the MakeByRefType method of type to convert the type to an instance that has this property set. – Martin Maat Jan 31 '20 at 10:25

Keith · Answer 2 · 2008-10-21T21:10:49.910

17

Yes. When you invoke the method pass the parameters that match the overload that you want.

For instance:

Type tp = myInstance.GetType();

//call parameter-free overload
tp.InvokeMember( "methodName", BindingFlags.InvokeMethod, 
   Type.DefaultBinder, myInstance, new object[0] );

//call parameter-ed overload
tp.InvokeMember( "methodName", BindingFlags.InvokeMethod, 
   Type.DefaultBinder, myInstance, new { param1, param2 } );

If you do this the other way round(i.e. by finding the MemberInfo and calling Invoke) be careful that you get the right one - the parameter-free overload could be the first found.

edited Oct 21 '08 at 21:10

answered Oct 21 '08 at 21:05

Keith

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Interestingly enough, this didn't work. I passed no parameters to my parameterless method, and I still got an ambiguous call. – Wes P Oct 21 '08 at 21:08
How does this work with parameters of different types? Say I have two overloads where one takes a string and the other takes an int? – smaclell Oct 21 '08 at 21:10
Not a problem - the underlying types of the parameters are what is checked. – Keith Oct 21 '08 at 21:11
The thing to be careful of is when you have types that implicitly cast to one another - then .Net picks instance methods over inherited methods. – Keith Oct 21 '08 at 21:12
See this question for an example of that: http://stackoverflow.com/questions/154112 – Keith Oct 21 '08 at 21:13
Having read other responses, what's the preferred way to do do this? Should I rely on InvokeMember to check the underlying types? Or should I make the call through Type.GetMethod, and explicitly type my parameters? I would think the later saves the runtime a bit of work... ? – Wes P Oct 22 '08 at 13:13
If you're calling it once just use mine, as it's slightly less complex. If you're doing it many times use @Hallgrim's and cache the MethodInfo of the overload you want. – Keith Oct 22 '08 at 13:16

score 5 · Answer 3 · answered Oct 21 '08 at 21:09

5

Use the GetMethod overload that takes a System.Type[], and pass an empty Type[];

typeof ( Class ).GetMethod ( "Method", new Type [ 0 ] { } ).Invoke ( instance, null );

answered Oct 21 '08 at 21:09

baretta

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You can use Type.EmptyTypes – yoel halb Apr 11 '14 at 15:33

How to use Reflection to Invoke an Overloaded Method in .NET

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