在 c# 中实现字典

Question

I was asked in an interview to implement the dictionary.How to implement it?I have tried implementing it using index as key using array.But Was unable to implement generic dictionary.

Answer 1

There are many ways to implement a class like Dictionary<T1, T2> .
I will describe a simple one.

1. Create a class and a List that stores all contents.
   [Edit] We will compare variables T1 values so a restriction, where T1: IEquatable<T1> is required.

class MyDictionary<T1, T2> where T1 : IEquatable<T1>
{
    private List<(T1 key, T2 val)> m_internal_data;
}

2. Implement a function that find a value inside the class.
   [Edit] Use Equals function. Using == causes an error.

public T2 Find(T1 key)
{
    // Looking for a content.
    foreach (var content in m_internal_data)
    {
        if (content.key.Equals(key))
        {
            return content.val;
        }
    }
    // It reaches here when there is no content which has the same key.
    // Then, I recommend to throw an exception or return a default value of T2.
    return default(T2);
}

3. Implement a function that assign a value.
   [Edit] Use Equals too.

public void Store(T1 key, T2 val)
{
    // Looking for a content. If exists, store a new value.
    for (int i = 0; i < m_internal_data.Count; i++)
    {
        if (m_internal_data[i].key.Equals(key))
        {
            var content = m_internal_data[i];
            content.val = val;
            m_internal_data[i] = content;
            return;
        }
    }
    // Create a new key.
    m_internal_data.Add((key, val));
}

4. Make it able to access a value by using square brackets. Just call previous functions.

public T2 this[T1 key]
{
    get => Find(key);
    set
    {
        Store(key, value);
    }
}

That's it.

Of course, this is not highly-optimized and has few useful functions. If you want to know how to write Dictionary more useful, I recommend you to read GitHub dotnet/runtime Dictionary.cs , which is included in .NET Core.