Question

Possible Duplicate:
Implicit typing; why just local variables?

c# allows this:

public class MyClass
{
    public void Foo()
    {
        var q = new MyObject();
    }
}

But it won’t allow this:

public class MyClass
{
    var q = new MyObject();

    public void Foo()
    {
        // ...
    }
}

Is there a reason for this? Thanks.

Answer 1

You can only use type inference for local variables.

The var keyword infers types for variables “that are declared at method scope”. In terms of the language, var is used to define an “implicitly typed local variable declaration” (C# Language Spec 8.5.1). The language only uses var for local variables, not type-level variables.

As to why, it’s a matter of how the C# designers intended it. It was not intended to infer types for all usages, only for a specific scenario. Eric Lippert described why this was done, providing many potential pitfalls that expanding the usage would cause, including potentially publically exposing an anonymous type, chaining multiple initializers, and more.