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How can var know of an undefined type?

Royce Roy 2216 03-Feb-2014

How can the implicit type variable var know a type that is not defined in the scope (using using)?

Example:

This is ok

public class MyClass
{
    public void MyMethod       
    {
        var list = AStaticClass.GetList();
    }
}

But this is not ok

public class MyClass
{
    public void MyMethod       
    {
        List<string> list = AStaticClass.GetList();
    }
}

In the last code snippet I have to add using System.Collections.Generic; for it to work.

How does this work?

c# c# 
Updated on 03-Feb-2014
Royce Roy

Royce Roy

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1 Answers

Pravesh Singh

Pravesh Singh

03-Feb-2014
Hi Royce,

When the compiler does the type inference it replaces var with System.Collections.Generic.List<string> and your code becomes:

public class MyClass
{
    public void MyMethod       
    {
        System.Collections.Generic.List<string> list = AStaticClass.GetList();
    }
}

But since the compiler spits IL, the following C# program (without any using statements):

public class Program
{
    static void Main()
    {
        var result = GetList();
    }
    static System.Collections.Generic.List<string> GetList()
    {
        return new System.Collections.Generic.List<string>();
    }
}

and the Main method looks like this:

.method private hidebysig static void Main() cil managed
{
    .entrypoint
    .maxstack 8
    L_0000: call class [mscorlib]System.Collections.Generic.List`1<string> Program::GetList()
    L_0005: pop
    L_0006: ret
}

As you can see the compiler inferred the type from the right hand-side of the assignment operator and replaced var with the fully qualified type name.


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