why var cannot be applied to member variable [duplicate]

This question already has answers here: Closed 12 years ago.

Possible Duplicates:

Why class fields cannot be var?

Implicit typing; why just local variables?

var in C# - Why开发者_JS百科 can't it be used as a member variable?

the var type cannot be applied to the member variables. I want to know why?

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Eric Lippert has a detailed blog post about this: Why no var on fields?

Summary:

• Even if it were feasible, it would require major compiler upheaval
• Determining the type could be very complex in some cases, and end up with some weird situations - what would be exposed for a public variable using an anonymous type?

I'd love it for simple cases, but the balance of "simple" may well be hard to decide in an elegant way which "feels right".

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var can only be used in a local scope. The actual type must be inferred by the compiler and it is too hard/impossible to determine the type from a construct like this:

class Person
{
    public var Name {get; set;}
}

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To put it simply :

// code fragment

int Add(int x, int y) { return x + y; }

var x = Add(int x, int y); // the compiler will know that the result of the call will be of type int


class X
{
    var x; // illegal, the compiler has no idea what's the target type of X
}

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Because it's type is not known. This is related to build order of compiler.

You can also have a look at this topic.