Repository navigation
Implicit capture fails with nullable value and @ symbol #2573
Description
private static void MyFunction<T>(T callback) where T : delegate void()
{
}
public static void Main()
{
let nullableInt = (int?)123;
if (let int1 = nullableInt)
{
// This call compiles successfully.
MyFunction(() => Console.WriteLine(int1));
}
if (let @int2 = nullableInt)
{
// Error: "Failed to lookup implicit capture `int2`"
MyFunction(() => Console.WriteLine(@int2));
}
let @int3 = 123;
// This call also compiles successfully.
MyFunction(() => Console.WriteLine(@int3));
}Tested on 10/15/2026. As shown, implicit capture fails in the second example, where the nullable integer is cast to a plain integer (within the if statement), but only when the variable is prefixed with @. The third example demonstrates that not all @ variables fail to capture.
If you're wondering why I'd bother using @ in the first place, I discovered this issue when using a nullable enum value and attempting a similar setup with a variable named @enum. I first assumed that the issue was because enum is a reserved keyword, but the code above demonstrates that that assumption was wrong.
let e = MyEnum.ABC;
if (let @enum = e)
{
// Error: "Failed to lookup implicit capture 'enum'"
MyFunction(() => Console.WriteLine(@enum));
}
Activity