避免进行不必要的强制转换

更新：2007 年 11 月

TypeName	DoNotCastUnnecessarily
CheckId	CA1800
类别	Microsoft.Performance
是否重大更改	否

原因

某方法对其参数或局部变量之一执行重复的强制转换。要通过该规则进行全面分析，必须使用调试信息生成被测试的程序集，并且关联的程序数据库 (.pdb) 文件必须可用。

规则说明

重复强制转换会降低性能，特别是在精简的迭代语句中执行强制转换时。对于显式重复强制转换操作，将强制转换的结果存储在局部变量中，并使用局部变量来替代重复强制转换操作。

如果在执行实际的强制转换之前，使用 C# is 运算符来测试强制转换是否将成功，请考虑转为测试 as 运算符的结果。后者可以提供相同的功能，但不需要由 is 运算符执行的隐式强制转换操作。

如何修复冲突

要修复与该规则的冲突，请修改方法实现，以最大限度地减少强制转换操作的次数。

何时禁止显示警告

如果无需顾虑性能，则可以安全地禁止显示此规则发出的警告，或者完全忽略此规则。

示例

下面的示例演示一个使用 C# is 运算符的与规则冲突的方法。第二个方法将 is 运算符替换为针对 as 运算符的结果进行的测试（该测试将每个迭代的强制转换操作的次数从两次减少为一次），从而满足该规则。

using System;
using System.Collections;
using System.Windows.Forms;

namespace PerformanceLibrary
{
   public sealed class SomeClass
   {
      private SomeClass() {}

      // This method violates the rule.
      public static void UnderPerforming(ArrayList list)
      {
         foreach(object obj in list) 
         {
            // The 'is' statement performs a cast operation.
            if(obj is Control) 
            {
               // The 'as' statement performs a duplicate cast operation.
               Control aControl = obj as Control;
               // Use aControl.
            }

         }
      }

      // This method satisfies the rule.
      public static void BetterPerforming(ArrayList list)
      {
         foreach(object obj in list) 
         {
            Control aControl = obj as Control;
            if(aControl != null) 
            {
               // Use aControl.
            }
         }
      }
   }
}

下面的示例演示一个与规则冲突的方法 start_Click（多次重复执行显式强制转换）和一个满足规则的方法 reset_Click（将强制转换存储在局部变量中）。

Imports System
Imports System.Drawing
Imports System.Windows.Forms

Namespace PerformanceLibrary

   Public Class SomeForm : Inherits Form

      Dim start, reset As Button

      Sub New()

         start = New Button()
         reset = New Button()
         AddHandler start.Click, AddressOf start_Click
         AddHandler reset.Click, AddressOf reset_Click
         Controls.Add(start)
         Controls.Add(reset)

      End Sub

      ' This method violates the rule.
      Private Sub start_Click(sender As Object, e As EventArgs)

         Dim controlSize As Size = DirectCast(sender, Control).Size
         Dim rightToLeftValue As RightToLeft = _ 
            DirectCast(sender, Control).RightToLeft
         Dim parent As Control = DirectCast(sender, Control)

      End Sub

      ' This method satisfies the rule.
      Private Sub reset_Click(sender As Object, e As EventArgs)

         Dim someControl As Control = DirectCast(sender, Control)
         Dim controlSize As Size = someControl.Size
         Dim rightToLeftValue As RightToLeft = someControl.RightToLeft
         Dim parent As Control = someControl

      End Sub

   End Class

End Namespace

using System;
using System.Drawing;
using System.Windows.Forms;

namespace PerformanceLibrary
{
   public class SomeForm : Form
   {
      Button start, reset;

      public SomeForm()
      {
         start = new Button();
         reset = new Button();
         start.Click += new EventHandler(start_Click);
         reset.Click += new EventHandler(reset_Click);
         Controls.Add(start);
         Controls.Add(reset);
      }

      // This method violates the rule.
      void start_Click(object sender, EventArgs e)
      {
         Size controlSize = ((Control)sender).Size;
         RightToLeft rightToLeftValue = ((Control)sender).RightToLeft;
         Control parent = (Control)sender;
      }

      // This method satisfies the rule.
      void reset_Click(object sender, EventArgs e)
      {
         Control someControl = (Control)sender;
         Size controlSize = someControl.Size;
         RightToLeft rightToLeftValue = someControl.RightToLeft;
         Control parent = someControl;
      }
   }
}

请参见

参考

as（C# 参考）

is（C# 参考）