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Using Bcl ImmutableDictionary in a Private Field

Let's say I have a class that will be called from multiple threads, and I'm going to store some data in an ImmutableDictionary in a private field in this class

public class Something {
    private ImmutableDictionary<string,string> _dict;
    public Something() {
       _dict = ImmutableDictionary<string,string>.Empty;
    }

    public void Add(string key, string value) {

       if(!_dict.ContainsKey(key)) {
          _dict = _dict.Add(key,value);
       }
    }
}

Could this be caused in this way by multiple threads that you get an error message that already exists in the dictionary?

Dictionary Thread1 checks false. Dictionary Thread2 checks false. Thread1 adds a value and the link to _dict is updated. Thread2 adds a value, but is it already added because it uses the same link?

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3 answers

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using System.Threading;

public class Something {
    private ImmutableDictionary<string, string> dict = ImmutableDictionary<string, string>.Empty;

    public void Add(string key, string value) {
       // It is important that the contents of this loop have no side-effects
       // since they can be repeated when a race condition is detected.
       do {
          var original = _dict;
          if (local.ContainsKey(key)) {
             return;
          }

          var changed = original.Add(key,value);
          // The while loop condition will try assigning the changed dictionary
          // back to the field. If it hasn't changed by another thread in the
          // meantime, we assign the field and break out of the loop. But if another
          // thread won the race (by changing the field while we were in an 
          // iteration of this loop), we'll loop and try again.
       } while (Interlocked.CompareExchange(ref this.dict, changed, original) != original);
    }
}

, , :

/// <summary>
/// Optimistically performs some value transformation based on some field and tries to apply it back to the field,
/// retrying as many times as necessary until no other thread is manipulating the same field.
/// </summary>
/// <typeparam name="T">The type of data.</typeparam>
/// <param name="hotLocation">The field that may be manipulated by multiple threads.</param>
/// <param name="applyChange">A function that receives the unchanged value and returns the changed value.</param>
public static bool ApplyChangeOptimistically<T>(ref T hotLocation, Func<T, T> applyChange) where T : class
{
    Requires.NotNull(applyChange, "applyChange");

    bool successful;
    do
    {
        Thread.MemoryBarrier();
        T oldValue = hotLocation;
        T newValue = applyChange(oldValue);
        if (Object.ReferenceEquals(oldValue, newValue))
        {
            // No change was actually required.
            return false;
        }

        T actualOldValue = Interlocked.CompareExchange<T>(ref hotLocation, newValue, oldValue);
        successful = Object.ReferenceEquals(oldValue, actualOldValue);
    }
    while (!successful);

    Thread.MemoryBarrier();
    return true;
}

"" :

public class Something {
    private ImmutableDictionary<string, string> dict = ImmutableDictionary<string, string>.Empty;

    public void Add(string key, string value) {
       ApplyChangeOptimistically(
          ref this.dict,
          d => d.ContainsKey(key) ? d : d.Add(key, value));
    }
}
+3

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