Lets assume that Foo is declared in class Test such as
public class Test
{
public void Foo<T>() { ... }
}
You need to first instantiate the method for type bar
using MakeGenericMethod. And then invoke it using reflection.
var mi = typeof(Test).GetMethod("Foo");
var fooRef = mi.MakeGenericMethod(bar);
fooRef.Invoke(new Test(), null);