開始試煉

自訂class 如何算等於
首先 先來自訂class

class Employee
{
    public int Id { get; set; }
    public string Name { get; set; }
}
void Main()
{
    var a = new Employee { Id = 1, Name = "a" };
    var b = new Employee { Id = 1, Name = "a" };
    a.Equals(b).Dump("a.Equals(b)");
    (a == b).Dump("a == b");

    var c = new List<Employee> { a, b };
    var d = c.Distinct().Dump();
}

可以看到不管 怎樣的等於 跟 用LINQ Distinct
Employee class 都無法算等於

開始 VS2022 來幫忙吧
先點快速動作與重構
選產生Equals 與 GetHashCode...

程式碼就產好了

class Employee : IEquatable<Employee>
{
    public int Id { get; set; }
    public string Name { get; set; }

    public override bool Equals(object obj)
    {
        return Equals(obj as Employee);
    }

    public bool Equals(Employee other)
    {
        return !(other is null) &&
               Id == other.Id &&
               Name == other.Name;
    }

    public override int GetHashCode()
    {
        int hashCode = -1919740922;
        hashCode = hashCode * -1521134295 + Id.GetHashCode();
        hashCode = hashCode * -1521134295 + EqualityComparer<string>.Default.GetHashCode(Name);
        return hashCode;
    }

    public static bool operator ==(Employee left, Employee right)
    {
        return EqualityComparer<Employee>.Default.Equals(left, right);
    }

    public static bool operator !=(Employee left, Employee right)
    {
        return !(left == right);
    }
}

整理重點
實做 IEquatable<T> 這樣就不用原生的
public override bool Equals(object obj)
減少裝箱拆箱的問題

實做 operator 的 == 跟 !=
讓 自訂物件也可以用 Equals的邏輯做等於比對

GetHashCode 會用到的地方還挺多的 基本上就是算出這class唯一的數字
像是dictionary 當key時就會用到 GetHashCode 來比對

最後要比對都會是呼叫
public bool Equals(Employee other)
然後就看比對邏輯就可以自己寫囉

延伸試煉

C# GetHashCode()系列文

結束試煉

現在這些IDE都會幫忙產生了 真方便