Addition (i.e. +) extension method with a generic C# Dictionary

I'm trying to write a generic Dictionary extension method like so. TValue will always be an int but TKey could be a string or a DateTime.

public static void AddOrIncrement<TKey, TValue>(this Dictionary<TKey, TValue> dict, TKey key, TValue value) 
{
    if (!dict.ContainsKey(key))
    {
        dict.Add(key, value);
    }
    else
    {
        dict[key] += value;
    }
}

But this won't compile - Operator '+=' cannot be applied to operands of type 'TValue' and 'TValue'.

So then I try:

public static void AddOrIncrement<TKey, TValue>(this Dictionary&l开发者_Go百科t;TKey, TValue> dict, TKey key, TValue value) where TValue : int

but that doesn't compile either. So then I tried:

public static void AddOrIncrement<TKey, int>(this Dictionary<TKey, int> dict, TKey key, int value)

which doesn't compile - "Type parameter declaration must be an identifier not a type"

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Try:

public static void AddOrIncrement<TKey>(this Dictionary<TKey, int> dict, TKey key, int value)

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You're trying to make an extension method that takes only one generic parameter:

public static void AddOrIncrement<TKey>(this Dictionary<TKey, int> dict, TKey key, int value)