Anytime you're writing code and you have to read it from left-to-right then right-to-left and back to left-to-right (aka all over the place), there should be an extension method to make it so that you can read everything from left-to-right. (L2R)

Here are the current estensions:

// See PHP's implode()
Array.Implode(string glue)

// Type casting
Object.CastAs<T>() // replaces (string)Object
Object.IsA<T>()
Object.IsAn<T>() // myNumber.IsAn<int>() instead of int.TryParse()

// Validation
Object.IsNotA<T>() / Object.IsNotA<T>()
Object.IsNull() / Object.IsNotNull()

// Arrays
Object.In(array); // inverse of Array.Contains
Object.NotIn(array); // inverse of !Array.Contains

Please add more as you think of them. BUT make sure to TDD it because even though the functions are simple, I want to make sure that we don't mess everything up by going fast.