Timespan in milliseconds to minutes and seconds only

I do it this way

timespan.ToString("mm\\:ss");

Reed's answer is ALMOST correct, but not quite. For example, if timespan is 00:01:59, Reed's solution outputs "2:59" due to rounding by the F0 numeric format. Here's the correct implementation:

string output = string.Format("{0}:{1:00}", 
        (int)timespan.TotalMinutes, // <== Note the casting to int.
        timespan.Seconds); 

In C# 6, you can use string interpolation to reduce code:

var output = $"{(int)timespan.TotalMinutes}:{timespan.Seconds:00}";

You can format this yourself using the standard numeric format strings:

string output = string.Format("{0}:{1}", (int)timespan.TotalMinutes, timespan.Seconds);