Concurrent Dictionary AddOrUpdate vs Index Add

Question

There are two ways I\'ve assigned values to a existing key in a concurrent dictionary in my current project.

A. concurrentDictionary1[key] = value;

Answer 1

If I know that the 'key' exists, are these functionally equivalent?

The way you're using it, yes. In fact, they're equivalent whether key exists or not.

What are the reasons for choosing one over the other?

AddOrUpdate accepts a function to be used to update the value. You're just using it to set the value directly, but it is designed to be used to update the value in a concurrent fashion based on the result of a function. For example:

concurrentDictionary2.AddOrUpdate(key, value, (k, v) => v + value);  // adds value to the existing value