我有一个类型为IEnumerable<KeyValuePair<T的对象,U>> keyValueList,我正在使用
var getResult= keyValueList.SingleOrDefault();
if(getResult==/*default */)
{
}
else
{
}
如何检查getResult是否为默认值,以防我找不到正确的元素?
我不能检查它是否为空,因为KeyValuePair是一个结构体。
我有一个类型为IEnumerable<KeyValuePair<T的对象,U>> keyValueList,我正在使用
var getResult= keyValueList.SingleOrDefault();
if(getResult==/*default */)
{
}
else
{
}
如何检查getResult是否为默认值,以防我找不到正确的元素?
我不能检查它是否为空,因为KeyValuePair是一个结构体。
当前回答
你可以创建一个通用的扩展方法,就像这样:
public static class Extensions
{
public static bool IsDefault<T>(this T value) where T : struct
{
bool isDefault = value.Equals(default(T));
return isDefault;
}
}
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'
其他回答
if(getResult.Key.Equals(default(T)) && getResult.Value.Equals(default(U)))
为了避免KeyValuePair.Equals(object)的装箱,你可以使用ValueTuple。
if ((getResult.Key, getResult.Value) == default)
试试这个:
if (getResult.Equals(new KeyValuePair<T,U>()))
或:
if (getResult.Equals(default(KeyValuePair<T,U>)))
我推荐更理解的方法使用扩展方法:
public static class KeyValuePairExtensions
{
public static bool IsNull<T, TU>(this KeyValuePair<T, TU> pair)
{
return pair.Equals(new KeyValuePair<T, TU>());
}
}
然后使用:
var countries = new Dictionary<string, string>
{
{"cz", "prague"},
{"de", "berlin"}
};
var country = countries.FirstOrDefault(x => x.Key == "en");
if(country.IsNull()){
}
你可以创建一个通用的扩展方法,就像这样:
public static class Extensions
{
public static bool IsDefault<T>(this T value) where T : struct
{
bool isDefault = value.Equals(default(T));
return isDefault;
}
}
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'