c# dictionary 给某键值赋值之前是否一定要add此键值

c# dictionary 给某键值赋值之前是否一定要add此键值？

Dictionary<keyType, valueType> dict = new ....;

void SetKeyValue(key, value)
{
    if (!dict.Contain(key))
        dict.Add(key, value);
    else
        dict[key] = value;
}