Avoid “Use of unassigned local variable” error

问题:

I have a two methods that are equivalent to this (pardon the contrived example):

public void WithResource(Action<Resource> action) {
    using (var resource = GetResource()) {
        action(resource);
    }
}

public void Test() {
    int id;
    SomeObject someObject;

    WithResource((resource) => {
        id = 1;
        someObject = SomeClass.SomeStaticMethod(resource);
    });

    Assert.IsNotNull(someObject);
    Assert.AreEqual(id, someObject.Id);
}

(There's some more logic in the WithResource call I'm trying to factor out.)

I'm getting Use of unassigned local variable compile-time errors because the assertions are... using unassigned variables. I'm currently avoiding the issue by assigning them -1 and null respectively.

Initializing to null doesn't feel bad, but I'd like to avoid putting the -1 in there... I would really like to tell the compiler "trust me, this will become initialized". Since it's a test, I don't actually care too much if it bombs, because that only means I'll have to fix the test.

I'm tempted to ask if there's a way to give that hint to the compiler, but have the feeling that's even uglier. Does such a hint exist or should I just initialize the variables like I'm doing now?

回答1:

Compiler is not smart enough to determine if the assignment would be made and hence the error.

You can't do anything about it, You have to assign it some default value, probably 0,-1, default(int) or int.MinValue

回答2:

You should initialize the variables. The compiler will never trust you ;)

回答3:

The other answers are correct. The easiest way to solve this problem is to initialize the locals.

I assume that you understand why the error is being produced: the compiler has no ability to know that the method called actually runs the lambda, and therefore no knowledge that the locals are initialized.

The only way to trick the compiler into not checking whether a variable is assigned is to make the variable non-local:

public void Test() {
    int[] id = new int[1];
    SomeObject[] someObject = new SomeObject[1];

    WithResource((resource) => {
        id[0] = 1;
        someObject[0] = SomeClass.SomeStaticMethod(resource);
    });

    Assert.IsNotNull(someObject[0]);
    Assert.AreEqual(id[0], someObject.Id);
}

Now you might say, well here I've clearly assigned id. Yes, but notice that the compiler does not complain that you've used id[0] before initializing it! The compiler knows that array element variables are initialized to zero.