Question

I am writing a custom dictionary class for my task but it is giving error. What is the problem at this class ? Thank you

        public struct MyValue
    {
        public int irValue1;
        public int irValue2;
    }

    public class csCustomDictionary : Dictionary<string, MyValue>
    {
        public void Add(string srKey, int irVal1, int irVal2)
        {
            if (this.ContainsKey(srKey) == true)
            {
                this[srKey].irValue1 = this[srKey].irValue1 + irVal1;
                this[srKey].irValue1 = this[srKey].irValue2 + irVal2;
            }
            else
            {
                MyValue val;
                val.irValue1 = irVal1;
                val.irValue2 = irVal2;
                this.Add(srKey, val);
            }
        }
    }
}

this is the error message

C# 4.0

Is this modified version correct

    public class csMyValue
{
    public int irValue1;
    public int irValue2;
}

public class csCustomDictionary : Dictionary<string, csMyValue>
{
    public void Add(string srKey, int irVal1, int irVal2)
    {
        if (this.ContainsKey(srKey) == true)
        {
            this[srKey].irValue1 = this[srKey].irValue1 + irVal1;
            this[srKey].irValue1 = this[srKey].irValue2 + irVal2;
        }
        else
        {
            csMyValue val = new csMyValue();
            val.irValue1 = irVal1;
            val.irValue2 = irVal2;
            this.Add(srKey, val);
        }
    }
}

Answer 1

The problem is that you’re using a struct as your Value in the Dictionary.

When you write: this[srKey], you actually retrieve a copy of the struct stored as the Dictionary’s value. As such, the .irValue1 is trying to set a field on the copy (that will soon disappear).

If you change MyValue to be a class, this should work. This is more appropriate, as well, as you’re making a mutable type, and mutable structs are very rarely a good idea.

That being said, you could force this to work via making a temporary, setting the fields, then setting the value in your Dictionary:

if (this.ContainsKey(srKey) == true)
{
    MyValue tmp = this[srKey];
    tmp.irValue1 += irVal1;
    tmp.irValue2 += irVal2;
    this[srKey] = tmp;
}

If you change MyValue to be a class instead of a struct, you could write:

    MyValue tmp = this[srKey];
    tmp.irValue1 += irVal1;
    tmp.irValue2 += irVal2;

Since the stored value would be a reference, this will do the change that you wish without requiring lots of dictionary lookups.