Question

I have this snippet.

List<Frames> FrameList;

where Frames is a class containing only primitives including a string field “ExerciseID”.

...


void GetFramesForExercise(string exerciseID)

    ....

    if (exerciseID == "3.2.2") { 
       Console.Write(""); }  // quick and dirty to add a breakpoint

    if (FramesList[115].ExerciseID.Equals(exerciseID)) { 
       Console.Write(""); } // quick and dirty to add a breakpoint

    frames = (Frames)FramesList.Single(r => r.ExerciseID.Equals(exerciseID));

By putting breakpoints on the console.write statements, I am able to see that exerciseID does indeed equal “3.2.2” and that FramesList[115] points to an instance of Exercise with ID equal to “3.2.2”. The instance pointed to is correctly initiliased.

Why does my query throw an InvalidOperationException?

Answer 1

If there are more than one matching elements, Single will throw an InvalidOperationException. (As you’ve checked there is at least one that matches, this is the only reason I can see that you’d get this exception.)

See the Exceptions section of this page.