Why can't we define a variable inside an if statement?

Question

Maybe this question has been answered before, but the word if occurs so often it\'s hard to find it.

The example doesn\'t make sense (the expression is

Answer 1

C# 7.0 introduced ability to declare out variables right inside conditions. In combination with generics, this can be leveraged for the requested result:

public static bool make (out T result) where T : new() {
    result = new T();
    return true;
}
// ... and later:
if (otherCondition && make(out var sb)) {
    sb.Append("hello!");
    // ...
}

You can also avoid generics and opt for a helper method instead:

public static bool makeStringBuilder(out StringBuilder result, string init) {
    result = new StringBuilder(init);
    return true;
}
// ... and later:
if (otherCondition && makeStringBuilder(out var sb, "hi!")) {
    sb.Append("hello!");
    // ...
}