String interning in .Net Framework - What are the benefits and when to use interning

In general, interning is something that just happens, automatically, when you use literal string values. Interning provides the benefit of only having one copy of the literal in memory, no matter how often it's used.

That being said, it's rare that there is a reason to intern your own strings that are generated at runtime, or ever even think about string interning for normal development.

There are potentially some benefits if you're going to be doing a lot of work with comparisons of potentially identical runtime generated strings (as interning can speed up comparisons via ReferenceEquals). However, this is a highly specialized usage, and would require a fair amount of profiling and testing, and wouldn't be an optimization I'd consider unless there was a measured problem in place.

Interning is an internal implementation detail. Unlike boxing, I do not think there is any benefit in knowing more than what you have read in Richter's book.

Micro-optimisation benefits of interning strings manually are minimal hence is generally not recommended.

This probably describes it:

class Program
{
    const string SomeString = "Some String"; // gets interned

    static void Main(string[] args)
    {
        var s1 = SomeString; // use interned string
        var s2 = SomeString; // use interned string
        var s = "String";
        var s3 = "Some " + s; // no interning 

        Console.WriteLine(s1 == s2); // uses interning comparison
        Console.WriteLine(s1 == s3); // do NOT use interning comparison
    }
}

This is an "old" question, but I have a different angle on it.

If you're going to have a lot of long-lived strings from a small pool, interning can improve memory efficiency.

In my case, I was interning another type of object in a static dictionary because they were reused frequently, and this served as a fast cache before persisting them to disk.

Most of the fields in these objects are strings, and the pool of values is fairly small (much smaller than the number of instances, anyway).

If these were transient objects, it wouldn't matter because the string fields would be garbage collected often. But because references to them were being held, their memory usage started to accumulate (even when no new unique values were being added).

So interning the objects reduced the memory usage substantially, and so did interning their string values while they were being interned.