DefaultIfEmpty(）
如果使用DefaultIfEmpty()的数组或列表为空，则返回元素类型的默认值。
MSDN
public static IEnumerable<TSource> DefaultIfEmpty<TSource>( this IEnumerable<TSource> source );
代码示例：
public static class Program
{
static void Main( string[] args )
{
int[] numbersA = new int[] {
0, 1, 2 };
int[] numbersB = new int[] {
};
IEnumerable<int> numbersAPlus = numbersA.DefaultIfEmpty();
IEnumerable<int> numbersBPlus = numbersB.DefaultIfEmpty();
System.Console.WriteLine( "numbersA :{0}", numbersA.Text() );
System.Console.WriteLine( "numbersAPlus :{0}", numbersAPlus.Text() );
System.Console.WriteLine( "numbersB :{0}", numbersB.Text() );
System.Console.WriteLine( "numbersBPlus :{0}", numbersBPlus.Text() );
System.Console.ReadKey();
}
public static string Text( this IEnumerable i_source )
{
string text = string.Empty;
foreach( var value in i_source )
{
text += string.Format( "[{0}], ", value );
}
return text;
}
}
numbersA :[0], [1], [2],
numbersAPlus :[0], [1], [2],
numbersB :
numbersBPlus :[0],
DefaultIfEmpty()可以将为空时返回的值通过参数传递。
MSDN
public static IEnumerable<TSource> DefaultIfEmpty<TSource>( this IEnumerable<TSource> source, TSource defaultValue );
代码示例：
public static class Program
{
static void Main( string[] args )
{
int[] numbersA = new int[] {
0, 1, 2 };
int[] numbersB = new int[] {
};
int defaultNumber = -1;
IEnumerable<int> numbersAPlus = numbersA.DefaultIfEmpty( defaultNumber );
IEnumerable<int> numbersBPlus = numbersB.DefaultIfEmpty( defaultNumber );
System.Console.WriteLine( "numbersA :{0}", numbersA.Text() );
System.Console.WriteLine( "numbersAPlus :{0}", numbersAPlus.Text() );
System.Console.WriteLine( "numbersB :{0}", numbersB.Text() );
System.Console.WriteLine( "numbersBPlus :{0}", numbersBPlus.Text() );
System.Console.ReadKey();
}
public static string Text( this IEnumerable i_source )
{
string text = string.Empty;
foreach( var value in i_source )
{
text += string.Format( "[{0}], ", value );
}
return text;
}
}
numbersA :[0], [1], [2],
numbersAPlus :[0], [1], [2],
numbersB :
numbersBPlus :[-1],
您还可以在自己的class中使用它，如下所示。
代码示例：
public static class Program
{
private class Parameter
{
public int ID {
get; set; }
public int Age {
get; set; }
public string Name {
get; set; }
public override string ToString()
{
return string.Format( "ID:{0}, Age:{1}, Name:{2}", ID, Age, Name );
}
}
static void Main( string[] args )
{
Parameter[] parametersA = new Parameter[]
{
new Parameter() {
ID = 0, Age = 52, Name = "正一郎" },
new Parameter() {
ID = 8, Age = 28, Name = "清次郎" },
new Parameter() {
ID = 3, Age = 20, Name = "誠三郎" },
new Parameter() {
ID = 4, Age = 18, Name = "征史郎" },
};
Parameter[] parametersB = new Parameter[] {
};
Parameter defaultparameter = new Parameter() {
ID = -1, Age = 0, Name = "无名" };
IEnumerable<Parameter> parametersAPlus = parametersA.DefaultIfEmpty( defaultparameter );
IEnumerable<Parameter> parametersBPlus = parametersB.DefaultIfEmpty( defaultparameter );
System.Console.WriteLine( "parametersA :{0}", parametersA.Text() );
System.Console.WriteLine( "parametersAPlus :{0}", parametersAPlus.Text() );
System.Console.WriteLine( "parametersB :{0}", parametersB.Text() );
System.Console.WriteLine( "parametersBPlus :{0}", parametersBPlus.Text() );
System.Console.ReadKey();
}
public static string Text( this IEnumerable i_source )
{
string text = string.Empty;
foreach( var value in i_source )
{
text += string.Format( "[{0}], ", value );
}
return text;
}
}
parametersA :[ID:0, Age:52, Name:正一郎], [ID:8, Age:28, Name:清次郎], [ID:3
, Age:20, Name:誠三郎], [ID:4, Age:18, Name:征史郎],
parametersAPlus :[ID:0, Age:52, Name:正一郎], [ID:8, Age:28, Name:清次郎], [ID:3
, Age:20, Name:誠三郎], [ID:4, Age:18, Name:征史郎],
parametersB :
parametersBPlus :[ID:-1, Age:0, Name:无名],