[Discussion] More statements as expressions.

[orthoxerox]

Since this was briefly mentioned in #5154 I've decided to create a separate issue for this discussion.

1. Blocks inside method bodies are expressions and return the value of their last expression.

let x = {
    let a = 1;
    let b = 2;
    a + b;
}

2. You cannot end a block inside a method body with a declaration or a statement that isn't guaranteed to not return if its return value is used.

let x = {
    let a = 1;
    let b = 2;
    a + b;
    int y; //I won't compile!
}

3. if is now an expression and accepts expressions as its two branches and returns the value of the one that is executed. When the return value of if is used its else branch becomes mandatory.

let x =
    if (list != null && list.Count > 0) {
        list[list.Count - 1];
    } else {
        getSomeDefaultValue();
    }

let y =
    if (list != null && list.Count > 0) {
        list[0];
    } //I won't compile!

4. for, foreach, while and do ... while are not expressions (because their body can be executed zero times).

5. try and its derivatives are now expressions and return the value of the try block when no exception is thrown and the value of a catch block when an exception is caught by one. The value of the finally block is discarded.

let result =
    try {
        GetResult();
    }
    catch (Exception ex) {
        logger.Log(ex);
        GetSomeDefaultResult();
    }
    finally {
        Cleanup();
    }

return lock(lockField)
    this.blahField;

6. switch is not an expression (because there's match.)

7. You cannot yield a value of a non-final expression in a block.

[alrz]

basically, #5402

[orthoxerox]

Duplicate of #5402