C# : Passing a Generic Object

Question

I want to have a generic print function...PrintGeneric(T)...in the following case, what am I missing?

As always your help/insight is appreciated...

public interface ITest
{}

public class MyClass1 : ITest
{
    public string myvar = "hello 1";
}

public class MyClass2 : ITest
{
    public string myvar = "hello 2";
}

class DoSomethingClass
{

    static void Main()
    {
        MyClass1 test1 = new MyClass1();
        MyClass2 test2 = new MyClass2();

        Console.WriteLine(test1.myvar);
        Console.WriteLine(test2.myvar);             
        Console.WriteLine(test1.GetType());

        PrintGeneric(test1);
        PrintGeneric<test2.GetType()>(test2);
    }

    // following doesn't compile
    public void PrintGeneric<T>(T test)
    {
        Console.WriteLine("Generic : " + test.myvar);
    }
}

`var` cannot be a variable name as it is a keyword. Can you show your actual code. — Ash Burlaczenko, Mar 12 '12 at 09:57
@AshBurlaczenko Not correct. `var` is a contextual keyword added in C# 3.0. You can use it as an identifier name. Similarly, you can use `yield`, `select`, `from`, etc. as identifier names in most contexts. — Mehrdad Afshari, Mar 12 '12 at 09:59
actually "var" did work without using just Console.WriteLine...but that was my bad to use a keyword as a identifier name... — user1229895, Mar 12 '12 at 10:01
In this case we managed to guess successfully where your problem lied and what the error was. However, you shouldn't rely on our psychic powers too much ;-) In general, it makes everyone's life easier if you precisely indicate the code line where the error occurs, and post the exact error message too. — Péter Török, Mar 12 '12 at 10:08

jonnystoten · Accepted Answer · 2012-03-12T10:13:43.173

32

It doesn't compile because T could be anything, and not everything will have the myvar field.

You could make myvar a property on ITest:

public ITest
{
    string myvar{get;}
}

and implement it on the classes as a property:

public class MyClass1 : ITest
{
    public string myvar{ get { return "hello 1"; } }
}

and then put a generic constraint on your method:

public void PrintGeneric<T>(T test) where T : ITest
{
    Console.WriteLine("Generic : " + test.myvar);
}

but in that case to be honest you are better off just passing in an ITest:

public void PrintGeneric(ITest test)
{
    Console.WriteLine("Generic : " + test.myvar);
}

edited Mar 12 '12 at 10:13

answered Mar 12 '12 at 09:59

jonnystoten

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I added a property to ITest, but then it tells me I need to implement it in MyClass1/MyClass2...which makes sense, but then I get an error saying it already contains a definition for myvar – user1229895 Mar 12 '12 at 10:11
Oh yeah, you need to then change the fields to be properties in the implementing classes. Updated my answer to show you. – jonnystoten Mar 12 '12 at 10:14
i tried PrintGeneric(test2) in Main for the generic and get an 'Only assignment, call, increment, decrement used as a statement' and PrintGeneric(test1) for PrintGeneric(ITest test)..and get a "An object reference is required for non-static field" – user1229895 Mar 12 '12 at 10:23
ok, i see I see I need to change to 'public static void PrintGeneric(ITest test)'...still unsure why PrintGeneric says only assignment/call/increment can be used as a statement – user1229895 Mar 12 '12 at 10:33

Jon Skeet · Answer 2 · 2012-03-12T10:11:03.077

8

You're missing at least a couple of things:

Unless you're using reflection, the type arguments need to be known at compile-time, so you can't use
```
PrintGeneric<test2.GetType()>
```
... although in this case you don't need to anyway
PrintGeneric doesn't know anything about T at the moment, so the compiler can't find a member called T

Options:

Put a property in the ITest interface, and change PrintGeneric to constrain T:

public void PrintGeneric<T>(T test) where T : ITest
{
    Console.WriteLine("Generic : " + test.PropertyFromInterface);
}

Put a property in the ITest interface and remove the generics entirely:

public void PrintGeneric(ITest test)
{
    Console.WriteLine("Property : " + test.PropertyFromInterface);
}

Use dynamic typing instead of generics if you're using C# 4

edited Mar 12 '12 at 10:11

answered Mar 12 '12 at 10:00

Jon Skeet

1,421,763
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1

Typo in your second code snippet there Jon? You said remove generics but there's still a on the method? – Steve Mar 12 '12 at 10:09
2

@SteveHaigh: Doh, that's what I get for typing and talking on the phone at the same time. Fixed, thanks. – Jon Skeet Mar 12 '12 at 10:11
trying the first option...I add "string myvar {get;}" to ITest... but get an error saying MyClass1/MyClass2 don't implement Interface member 'ITest.myvar'...adding "string myvar {get;}" to MyClass1/MyClass2 results in compile error saying its already defined? – user1229895 Mar 12 '12 at 10:16
@user1229895: You can't declare a concrete property like that... but that has nothing to do with generics. You can use an automatically implemented property to implement it in MyClass1 with `public string myvar { get; private set; }`. If that's not enough help, I suggest you ask a separate question. – Jon Skeet Mar 12 '12 at 10:21
As far as I know dynamic types are not recommended for basic functionalities like this. It should be your last resort. – Ozkan Jun 17 '20 at 09:33
1

@Ozkan: Well it is last in the list here... although I'd say that using dynamic typing instead of manually-written reflection can be useful to effectively offload the reflection code to a well-tested codebase. – Jon Skeet Jun 17 '20 at 11:33

Péter Török · Answer 3 · 2012-03-12T10:05:52.357

In your generic method, T is just a placeholder for a type. However, the compiler doesn't per se know anything about the concrete type(s) being used runtime, so it can't assume that they will have a var member.

The usual way to circumvent this is to add a generic type constraint to your method declaration to ensure that the types used implement a specific interface (in your case, it could be ITest):

public void PrintGeneric<T>(T test) where T : ITest

Then, the members of that interface would be directly available inside the method. However, your ITest is currently empty, you need to declare common stuff there in order to enable its usage within the method.

score 2 · Answer 4 · answered Mar 12 '12 at 10:00

You'll have to provide more information about the generic type T. In your current PrintGeneric method, T might as well be a string, which does not have a var member.

You may want to change var to a property rather than a field

public interface ITest
{
    string var { get; }
}

And add a constraint where T: ITest to the PrintGeneric method.

score 1 · Answer 5 · edited Mar 12 '12 at 10:08

1

try

public void PrintGeneric<T>(T test) where T: ITest
{
    Console.WriteLine("Generic : " + test.@var);
}

as @Ash Burlaczenko has said you cant name a variable after a keyword, if you reallllly want this prefix with @ symbol to escape the keyword

edited Mar 12 '12 at 10:08

Moo-Juice

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answered Mar 12 '12 at 09:58

undefined

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This will not work, `ITest` does not have a member named `var`. – Mehrdad Afshari Mar 12 '12 at 09:59
Only going to work if ITest specifies the property var, of course. I'm not sure it does in the example. – Alexander R Mar 12 '12 at 10:00
i get a number of errors... CS0120 : An Object Reference is Required for non-static field/method/property 'DoSomethingClass.PrintGeneric(T) CS0201 : Only Assignment, call, increment, decrement, new object can be used as a statement CS0019 : operator < cannote be applied to operands of type Method Group CS1061 : T does not contain definition myvar – user1229895 Mar 12 '12 at 10:04

Moo-Juice · Answer 6 · 2012-03-12T10:08:50.177

1

You need to define something in the interface, such as:

public interface ITest
{
    string Name { get; }
}

Implement ITest in your classes:

public class MyClass1 : ITest
{
    public string Name { get { return "Test1"; } }
}

public class MyClass2 : ITest
{
    public string Name { get { return "Test2"; } }
}

Then restrict your generic Print function, to ITest:

public void Print<T>(T test) where T : ITest
{
}

edited Mar 12 '12 at 10:08

answered Mar 12 '12 at 10:01

Moo-Juice

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See my reply on Ash's comment to the question. You can use `var` as identifier name, just like `yield`, etc., though that's probably not a great idea. It's a contextual keyword. Try `int var;` to verify for yourself. – Mehrdad Afshari Mar 12 '12 at 10:06
@MehrdadAfshari, ah.. I was not aware of that. Well, today I learned. But yes, it's not something I'd ever actually do :) – Moo-Juice Mar 12 '12 at 10:08

score 0 · Answer 7 · answered Mar 12 '12 at 10:01

0

You cannot access var with the generic.

Try something like

Console.WriteLine("Generic : {0}", test);

And override ToString method [1]

[1] http://msdn.microsoft.com/en-us/library/system.object.tostring.aspx

answered Mar 12 '12 at 10:01

nolith

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C# : Passing a Generic Object

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