Breaking out of a foreach loop from within a switch block

The boolean is one way. Another is using labels and goto. I know folks consider goto to be a cardinal sin, but used judiciously (VERY judiciously), it can be useful. In this case, place a label just past the end of the foreach loop. When you want to exit the loop, simply goto that label. For example:

foreach(var v in myCollection) {
    switch(v.Id) {
        case 1:
            if(true) {
                goto end_foreach;
            }
            break;
        case 2:
            break;
    }
}
end_foreach:
// ... code after the loop

EDIT: some people have mentioned taking the loop out into a separate method so that you can use return. I see the benefit of this as it doesn't require goto and it also simplifies the original function that contained the loop. However, if the loop is simple and is the primary purpose of the function that contains it, or if the loop makes use of out or ref variables, then it's probably best to just leave it in place and use the goto. In fact, because the goto and the label stand out, it probably makes the code clearer rather than clunkier. Putting it in a separate function could make simple code harder to read.

Your solution is pretty much the most common option in this case. That being said, I'd put your exit check at the end:

bool exitLoop;
foreach (var v in myCollection)
{
    switch (v.id)
    {
        case 1:
            if (true)
            {
                exitLoop = true;
            }
            break;
        case 2;
            break
    }

    // This saves an iteration of the foreach...
    if (exitLoop) break;
}

The other main option is to refactor your code, and pull the switch statement and foreach loop out into a separate method. You could then just return from inside the switch statement.