c# - "Use of unassigned local variable" error even though I assign the variable?

Question

In the example below, the compiler generates an error "Use of unassigned local variable r", even though I assign the variable in loop before using it. Why does the compiler generate this error?

static void Main(string[] args)
{
    float r;

    for (int i = 0; i < 100; i++)
        r = i; // assigned here

    Console.WriteLine(r); // error: use of unassigned local variable            
}

Answer 1

The compiler generates that error whenever it detects an unassigned variable is possible.

Because the body of for loops are not guaranteed to execute—for example, for(int i = 123; i < 0; i++)—the variable is not guaranteed to be assigned, and so the compiler generates the error.

From the documentation:

The C# compiler does not allow the use of uninitialized variables. If the compiler detects the use of a variable that might not have been initialized, it generates compiler error CS0165. For more information, see Fields (C# Programming Guide). Note that this error is generated when the compiler encounters a construct that might result in the use of an unassigned variable, even if your particular code does not. This avoids the necessity of overly-complex rules for definite assignment.