C# KeyValuePair 的默认值
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/1641392/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
The default for KeyValuePair
提问by Graviton
I have an object of the type IEnumerable<KeyValuePair<T,U>> keyValueList, I am using
我有一个类型的对象IEnumerable<KeyValuePair<T,U>> keyValueList，我正在使用
var getResult= keyValueList.SingleOrDefault();
if(getResult==/*default */)
{
}
else
{
}
How can I check whether getResultis the default, in case I can't find the correct element?
getResult如果找不到正确的元素，如何检查是否为默认值？
I can't check whether it is nullor not, because KeyValuePairis a struct.
我无法检查它是否null存在，因为它KeyValuePair是一个结构体。
采纳答案by Andrew Hare
Try this:
尝试这个：
if (getResult.Equals(new KeyValuePair<T,U>()))
or this:
或这个：
if (getResult.Equals(default(KeyValuePair<T,U>)))
回答by ChaosPandion
if(getResult.Key.Equals(default(T)) && getResult.Value.Equals(default(U)))
回答by pholpar
You can create a general (and generic) extension method, like this one:
您可以创建一个通用（和通用）扩展方法，如下所示：
public static class Extensions
{
public static bool IsDefault<T>(this T value) where T : struct
{
bool isDefault = value.Equals(default(T));
return isDefault;
}
}
Usage:
用法：
// 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'
回答by Peter
Try this:
尝试这个：
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));
回答by Stephen Westlake - Infusion
From your original code it looks like what you want is to check if the list was empty:
从您的原始代码看来，您想要检查列表是否为空：
var getResult= keyValueList.SingleOrDefault();
if (keyValueList.Count == 0)
{
/* default */
}
else
{
}
回答by Miroslav Holec
I recommend more understanding way using extension method:
我推荐使用扩展方法的更多理解方式：
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:
然后只需使用：
var countries = new Dictionary<string, string>
{
{"cz", "prague"},
{"de", "berlin"}
};
var country = countries.FirstOrDefault(x => x.Key == "en");
if(country.IsNull()){
}
回答by Jonas Nyrup
To avoid the boxing of KeyValuePair.Equals(object)you can use a ValueTuple.
为避免拳击，KeyValuePair.Equals(object)您可以使用ValueTuple.
if ((getResult.Key, getResult.Value) == default)
