how to get hash of the object?

Question

I am read excel and write it to database.

for the duplicate item.

how to get the hash code of each item?

then I can compare it by the hash code.

public class MyClassA
{
public int Id{get;set;}
public string UserInfo{get;set;}
public decimal count{get;set;}
}

how to get the hash of it?

Answer 1

Try this:

public class MyClassA
{
    public int Id { get; set; }
    public string UserInfo { get; set; }
    public decimal count { get; set; }

    public override int GetHashCode( )
    {
        return HashCode.Combine( Id, UserInfo, count );
    }
}

// or this:

public struct MyStructA
{
    public int Id { get; set; }
    public string UserInfo { get; set; }
    public decimal count { get; set; }

    // Use the default 'GetHashCode()'. The hash code is calculated by .NET. 
}

The documentation recommends to override the Equals(object) function too.

By the way, if two objects are not equal, the hash codes are not always different. GetHashCode is not enough to find the duplicates.

Answer 2

Try an alternative, which is also suggested by Visual Studio wizards:

public class MyClassA
{
    public int Id { get; set; }
    public string UserInfo { get; set; }
    public decimal count { get; set; }

    public override int GetHashCode( )
    {
        int hashCode = -525015890;

        hashCode = hashCode * -1521134295 + Id.GetHashCode( );
        hashCode = hashCode * -1521134295 + EqualityComparer<string>.Default.GetHashCode( UserInfo );
        hashCode = hashCode * -1521134295 + count.GetHashCode( );

        return hashCode;
    }
}

Answer 3

this is not what I want

Answer 4

The IEqualityComparer<T> Interface might be solution. The following example gets a distinct list of objects.

https://learn.microsoft.com/en-us/dotnet/api/system.collections.generic.iequalitycomparer-1?view=net-8.0

using System.Diagnostics.CodeAnalysis;

List<MyClassA> items = new List<MyClassA>() 
{ 
    new MyClassA() 
    { 
        Id = 1,
        UserInfo = "User Info 1",
        count = 1
    },
    new MyClassA()
    {
        Id = 2,
        UserInfo = "User Info 2",
        count = 2
    },
    new MyClassA()
    {
        Id = 1,
        UserInfo = "User Info 1",
        count = 1
    },
};

var DistinctItems = items.Distinct(new MyClassAComparer());
foreach (var item in DistinctItems)
{
    Console.WriteLine($"{item.Id} {item.UserInfo} {item.count}");
}



public class MyClassAComparer : IEqualityComparer<MyClassA>
{
    public bool Equals(MyClassA? c1, MyClassA? c2)
    {
        if (ReferenceEquals(c1, c2))
            return true;

        if (c1 is null || c2 is null)
            return false;

        return c1.Id == c2.Id
            && c1.UserInfo == c2.UserInfo
            && c1.count == c2.count;
    }

    public int GetHashCode([DisallowNull] MyClassA v) => v.Id.GetHashCode()
        ^ v.UserInfo.GetHashCode()
        ^ v.count.GetHashCode();
}

public class MyClassA
{
    public int Id { get; set; }
    public string UserInfo { get; set; } = string.Empty;
    public decimal count { get; set; }
}

Answer 5

.net has a built in hash code function (.GetHashCode()) designed for hash table lookups. See

https://learn.microsoft.com/en-us/dotnet/fundamentals/runtime-libraries/system-object-gethashcode

note: as is true with any hash, two different values may have the same hash, so if you get a hash match, you still need to do a value match.