Can static methods be called using object/instance in .NET
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Published on Wed, 16 Dec 2009 04:47:00 +0000
Indexed on
2010/03/16
15:31 UTC
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Ans is Yes and No
Yes in C++, Java and VB.NET
No in C#
This is only compiler restriction in c#. You might see in some websites that we can break this restriction using reflection and delegates, but we can’t, according to my little research J
I shall try to explain you…
Following is code sample to break this rule using reflection, it seems that it is possible to call a static method using an object, p1
using System;
namespace T
{
class Program
{
static void Main()
{
var p1 = new Person() { Name = "Smith" };
typeof(Person).GetMethod("TestStatMethod").Invoke(p1, new object[] { });
}
class Person
{
public string Name { get; set; }
public static void TestStatMethod()
{
Console.WriteLine("Hello");
}
}
}
}
but I do not think so this method is being called using p1 rather Type Name “Person”. I shall try to prove this…
look at another example… Test2 has been inherited from Test1.
Let’s see various scenarios…
Scenario1
using System;
namespace T
{
class Program
{
static void Main()
{
Test1 t = new Test1();
typeof(Test2).GetMethod("Method1").Invoke(t,
new object[] { });
}
}
class Test1
{
public static void Method1()
{
Console.WriteLine("At test1::Method1");
}
}
class Test2 : Test1
{
public static void Method1()
{
Console.WriteLine("At test1::Method2");
}
}
}
Output:
At test1::Method2
Scenario2
static void Main()
{
Test2 t = new Test2();
typeof(Test2).GetMethod("Method1").Invoke(t,
new object[] { });
}
Output:
At test1::Method2
Scenario3
static void Main()
{
Test1 t = new Test2();
typeof(Test2).GetMethod("Method1").Invoke(t,
new object[] { });
}
Output:
At test1::Method2
In all above scenarios output is same, that means, Reflection also not considering the object what you pass to Invoke method in case of static methods. It is always considering the type which you specify in typeof().
So, what is the use passing instance to “Invoke”. Let see below sample
using System;
namespace T
{
class Program
{
static void Main()
{
typeof(Test2).GetMethod("Method1").
Invoke(null, new object[] { });
}
}
class Test1
{
public static void Method1()
{
Console.WriteLine("At test1::Method1");
}
}
class Test2 : Test1
{
public static void Method1()
{
Console.WriteLine("At test1::Method2");
}
}
}
Output is
At test1::Method2
I was able to call Invoke “Method1” of Test2 without any object. Yes, there no wonder here as Method1 is static. So we may conclude that static methods cannot be called using instances (only in c#)
Why Microsoft has restricted it in C#?
Ans: Really there Is no use calling static methods using objects because static methods are stateless.
but still Java and C++ latest compilers allow calling static methods using instances.
Java sample
class Test
{
public static void main(String str[])
{
Person p = new Person();
System.out.println(p.GetCount());
}
}
class Person
{
public static int GetCount()
{
return 100;
}
}
Output
100
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