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]);