Bug 17860 - New "Use of unassigned local variable" error in 3.2.7
Summary: New "Use of unassigned local variable" error in 3.2.7
Alias: None
Product: Compilers
Classification: Mono
Component: C# ()
Version: 3.2.x
Hardware: Macintosh Mac OS
: --- normal
Target Milestone: ---
Assignee: Marek Safar
Depends on:
Reported: 2014-02-18 17:24 UTC by Adam Patridge
Modified: 2014-02-19 12:44 UTC (History)
1 user (show)

Is this bug a regression?: ---
Last known good build:

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Description Adam Patridge 2014-02-18 17:24:32 UTC
After updating to Mono 3.2.7, a chunk of previously-compiling code started causing a compile error.

    public static Task<T> GetEventAsync<T>(this object eventSource, string eventName) where T : EventArgs {
        var tcs = new TaskCompletionSource<T>();
        var type = eventSource.GetType();
        var ev = type.GetEvent(eventName);

        EventHandler handler = (sender, e) => {
            ev.RemoveEventHandler(eventSource, handler); // <-- Error CS0165: Use of unassigned local variable `handler'

        ev.AddEventHandler(eventSource, handler);
        return tcs.Task;

This code is derived from code put out by Frank Krueger in a blog post (http://praeclarum.org/post/45231096776/await-in-the-land-of-ios-drag-n-drop), which also causes the same build error on 3.2.7, but I can't verify this code compiled previously.

    EventHandler handler;
    handler = delegate (object sender, EventArgs e) {
        ev.RemoveEventHandler(eventSource, handler);

Here's a workaround for as long as it is needed.

    EventHandler handler = null;
    handler = (sender, e) => {
        ev.RemoveEventHandler(eventSource, handler);
Comment 1 Marek Safar 2014-02-19 12:44:45 UTC
The compiler error is correct. The local variable is unassigned