📅 最后修改于: 2020-11-01 03:12:25 🧑 作者: Mango
C＃默认表达式是一个类(DefaultExpression)，用于表示空表达式的默认值。它是System.Linq.Expressions.Expression命名空间的子类。
为了获取表达式的默认值，Expression类提供了一个静态方法Default(Type)，该方法返回DefaultExpression类的实例。
public sealed class DefaultExpression : Expression
它提供以下属性和方法。
| Name | Description |
|---|---|
| CanReduce | This property is inherited from Expressionn. It Indicates that the node can be reduced to a simpler node. |
| NodeType | It is used to return the node type of this expression. |
| Type | Gets the static type of the expression that this Expression represents. |
| Name | Description |
|---|---|
| Accept(ExpressionVisitor) | It is used to dispatch the specific visit method for this node type. |
| Equals(Object) | It determines whether the specified object is equal to the current object or not. |
| GetHashCode() | This method serves as the default hash function. |
| GetType() | It is used to get the type of the current instance. |
| Reduce() | It is used to reduce this node to a simpler expression. |
| ReduceAndCheck() | It is used to reduce this node to a simpler expression by explicit checking. |
| ReduceExtensions() | It reduces the expression to a known node type. |
| ToString() | It is used to return a string representation of the Expression. |
| VisitChildren(ExpressionVisitor) | It reduces the node and then calls the visitor delegate on the reduced expression. |
Expression.Default()方法具有以下签名。
public static DefaultExpression Default(Type type)
它采用System.Type类型的参数，并返回DefaultExpression类的实例。
using System;
using System.Linq.Expressions;
namespace CSharpFeatures
{
class AsynchronousMethod
{
public static void Main(string[] args = default)
{
Expression defaultExpression = Expression.Default(typeof(int));
show(defaultExpression);
}
static void show(Expression defaultExpression)
{
// Default expression properties and methods
Console.WriteLine("Instace: " + defaultExpression);
Console.WriteLine("Type: " + defaultExpression.Type);
Console.WriteLine("Can reduce: " + defaultExpression.CanReduce);
Console.WriteLine("Instance type: " + defaultExpression.GetType());
Console.WriteLine("Node type: " + defaultExpression.NodeType);
}
}
}
输出：
Instace: default(Int32)
Type: System.Int32
Can reduce: False
Instance type: System.Linq.Expressions.DefaultExpression
Node type: Default