CODEWITHSUNDEEP
c#key-value
Graviton
Graviton

Reputation: 83254

The default for KeyValuePair

I have an object of the type IEnumerable<KeyValuePair<T,U>> keyValueList, I am using 

 var getResult= keyValueList.SingleOrDefault();
 if(getResult==/*default */)
 {
 }
 else
 {
 }

How can I check whether getResult is the default, in case I can't find the correct element?

I can't check whether it is null or not, because KeyValuePair is a struct.

Upvotes: 500

Views: 254491

Answers (9)

Supawat Pusavanno
Supawat Pusavanno

Reputation: 3366

using pattern match in C# 8.0 or above or .NET Core 3.x, .NET 5.x or above

//
// assume key is string and value is object
//

var getResult = keyValueList.SingleOrDefault();

// if clause
if (getResult is { Key: default(string), Value: default(object) })
{
    do_both_key_value_are_default();
}
// if clause negate
if (getResult is not { Key: default(string), Value: default(object) })
{
    do_both_key_value_found();
}


// switch style
switch (getResult)
{
    case { Key: default(string), Value: default(object) }: // this is default case where cannot find the single
        do_both_key_value_are_default();
        break;
    default:
        do_something_else();
        break;
}

Upvotes: 0

Caius Jard
Caius Jard

Reputation: 74605

It may suffice, for your application, to check if either the Key, or the Value are default.. For example if you have a KeyValuePair<int,string> mapping positive nonzero integers to strings, and the strings are never null, then either:

if(getResult.Key == default)

if(getResult.Value == default)

Would be an indicator that a default KVP has been returned

In essence, there may be no need to test the whole KVP for being default, if just one test will do; consider it for your use case

Upvotes: 3

Jonas Nyrup
Jonas Nyrup

Reputation: 2586

To avoid the boxing of KeyValuePair.Equals(object) you can use a ValueTuple.

if ((getResult.Key, getResult.Value) == default)

Upvotes: 8

ChaosPandion
ChaosPandion

Reputation: 78272

if(getResult.Key.Equals(default(T)) && getResult.Value.Equals(default(U)))

Upvotes: 10

Miroslav Holec
Miroslav Holec

Reputation: 3217

I recommend more understanding way using extension method:

public static class KeyValuePairExtensions
{
    public static bool IsNull<T, TU>(this KeyValuePair<T, TU> pair)
    {
        return pair.Equals(new KeyValuePair<T, TU>());
    }
}

And then just use:

var countries = new Dictionary<string, string>
{
    {"cz", "prague"},
    {"de", "berlin"}
};

var country = countries.FirstOrDefault(x => x.Key == "en");

if(country.IsNull()){

}

Upvotes: 10

pholpar
pholpar

Reputation: 1785

You can create a general (and generic) extension method, like this one:

public static class Extensions
{
    public static bool IsDefault<T>(this T value) where T : struct
    {
        bool isDefault = value.Equals(default(T));

        return isDefault;
    }
}

Usage:

// We have to set explicit default value '0' to avoid build error:
// Use of unassigned local variable 'intValue'
int intValue = 0;
long longValue = 12;
KeyValuePair<String, int> kvp1 = new KeyValuePair<String, int>("string", 11);
KeyValuePair<String, int> kvp2 = new KeyValuePair<String, int>();
List<KeyValuePair<String, int>> kvps = new List<KeyValuePair<String, int>> { kvp1, kvp2 };
KeyValuePair<String, int> kvp3 = kvps.FirstOrDefault(kvp => kvp.Value == 11);
KeyValuePair<String, int> kvp4 = kvps.FirstOrDefault(kvp => kvp.Value == 15);

Console.WriteLine(intValue.IsDefault()); // results 'True'
Console.WriteLine(longValue.IsDefault()); // results 'False'
Console.WriteLine(kvp1.IsDefault()); // results 'False'
Console.WriteLine(kvp2.IsDefault()); // results 'True'
Console.WriteLine(kvp3.IsDefault()); // results 'False'
Console.WriteLine(kvp4.IsDefault()); // results 'True'

Upvotes: 119

Stephen Westlake - Infusion
Stephen Westlake - Infusion

Reputation: 101

From your original code it looks like what you want is to check if the list was empty:

var getResult= keyValueList.SingleOrDefault();
if (keyValueList.Count == 0)
{
   /* default */
}
else
{
}

Upvotes: 9

Peter
Peter

Reputation: 329

Try this:

KeyValuePair<string,int> current = this.recent.SingleOrDefault(r => r.Key.Equals(dialog.FileName) == true);

if (current.Key == null)
    this.recent.Add(new KeyValuePair<string,int>(dialog.FileName,0));

Upvotes: 32

Andrew Hare
Andrew Hare

Reputation: 351526

Try this:

if (getResult.Equals(new KeyValuePair<T,U>()))

or this:

if (getResult.Equals(default(KeyValuePair<T,U>)))

Upvotes: 683

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