目录

一、定义

二、示例

三、生成

---

        在程序的开发过程中经常会使用类型转换，如果类型转换不成功则会出现异常，从抛出异常到捕获并处理异常，无形中增加了系统的开销，而且太过频繁地处理异常还会严重地影响系统的稳定性。is关键字可以有效地解决上面出现的问题。is关键字用于检查对象是否与给定类型兼容，如果兼容则返回true，如果不兼容则返回false。在进行类型转换前，可以先使用is关键字判断对象是否与指定类型兼容，如果兼容才进行转换，这样的转换是安全的。

一、定义

        is关键字主要用于检查对象是否与指定类型兼容，如果兼容则返回true，如果不兼容则返回false。

二、示例

// 使用is关键字检查对象是否与给定类型兼容
namespace _025
{
    public partial class Form1 : Form
    {
        private GroupBox? groupBox1;
        private GroupBox? groupBox2;
        private RadioButton? radioButton2;
        private RadioButton? radioButton1;
        private RadioButton? radioButton4;
        private RadioButton? radioButton3;
        private Button? button1;

        public Form1()
        {
            InitializeComponent();
            Load += Form1_Load;
        }
        private void Form1_Load(object? sender, EventArgs e)
        {
            // 
            // radioButton2
            // 
            radioButton2 = new RadioButton
            {
                AutoSize = true,
                Location = new Point(42, 49),
                Name = "radioButton2",
                Size = new Size(102, 21),
                TabIndex = 1,
                TabStop = true,
                Text = "文件对象",
                UseVisualStyleBackColor = true,
            };
            // 
            // radioButton1
            // 
            radioButton1 = new RadioButton
            {
                AutoSize = true,
                Location = new Point(42, 22),
                Name = "radioButton1",
                Size = new Size(102, 21),
                TabIndex = 2,
                TabStop = true,
                Text = "字符串对象",
                UseVisualStyleBackColor = true,
            };
            // 
            // groupBox1
            // 
            groupBox1 = new GroupBox
            {
                Location = new Point(0, 0),
                Name = "groupBox1",
                Size = new Size(150, 100),
                TabIndex = 0,
                TabStop = false,
                Text = "对象"
            };
            groupBox1.Controls.Add(radioButton2);
            groupBox1.Controls.Add(radioButton1);
            groupBox1.SuspendLayout();
            // 
            // radioButton4
            // 
            radioButton4 = new RadioButton
            {
                AutoSize = true,
                Location = new Point(42, 49),
                Name = "radioButton4",
                Size = new Size(102, 21),
                TabIndex = 1,
                TabStop = true,
                Text = "Fileinfo类型",
                UseVisualStyleBackColor = true,
            };
            // 
            // radioButton3
            // 
            radioButton3 = new RadioButton
            {
                AutoSize = true,
                Location = new Point(42, 22),
                Name = "radioButton3",
                Size = new Size(102, 21),
                TabIndex = 2,
                TabStop = true,
                Text = "string类型",
                UseVisualStyleBackColor = true,
            };
            // 
            // groupBox2
            //            
            groupBox2 = new GroupBox
            {
                Location = new Point(165, 0),
                Name = "groupBox2",
                Size = new Size(150, 100),
                TabIndex = 0,
                TabStop = false,
                Text = "类型"
            };
            groupBox2.Controls.Add(radioButton4);
            groupBox2.Controls.Add(radioButton3);
            groupBox2.SuspendLayout();            
            // 
            // button1
            // 
            button1 = new Button
            {
                Location = new Point(114, 116),
                Name = "button1",
                Size = new Size(75, 23),
                TabIndex = 1,
                Text = "查看是否兼容",
                UseVisualStyleBackColor = true
            };
            button1.Click += Button1_Click;
            // 
            // Form1
            // 
            AutoScaleDimensions = new SizeF(7F, 17F);
            AutoScaleMode = AutoScaleMode.Font;
            ClientSize = new Size(314, 151);
            Controls.Add(button1);
            Controls.Add(groupBox2);
            Controls.Add(groupBox1);
            Name = "Form1";
            StartPosition = FormStartPosition.CenterScreen;
            Text = "is关键字查看类型兼容";            
            groupBox1.ResumeLayout(false);
            groupBox1.PerformLayout();
            groupBox2.ResumeLayout(false);
            groupBox2.PerformLayout();
        }
        /// <summary>
        /// 容器1里的2个单选按钮只能二选一，要么字符串对象要么文件对象，
        /// 容器2里的2个单选按钮只能二选一，要么字符串类型要么文件类型，
        /// 当文件对象与文件类型配对检查时，判断为类型兼容，
        /// 当字符串对象与字符串类型配对检查时，判断为类型兼容，
        /// 其他配对情况均判断为类型不兼容。
        /// </summary>
        private void Button1_Click(object? sender, EventArgs e)
        {
            object P_obj = radioButton1!.Checked ? //正确的为变量添加引用
               "C# 编程词典" : new FileInfo(@"d:\");
            if (radioButton3!.Checked)  //判断选择了哪一个类型
            {
                if (P_obj is string)    //判断对象是否为字符串类型
                    MessageBox.Show(    //提示兼容信息
                        "对象与指定类型兼容", "提示！");
                else
                    MessageBox.Show(    //提示不兼容信息
                        "对象与指定类型不兼容", "提示！");
            }
            else
            {
                if (P_obj is FileInfo)  //判断对象是否为文件类型
                    MessageBox.Show(    //提示兼容信息
                        "对象与指定类型兼容", "提示！");
                else
                    MessageBox.Show(    //提示不兼容信息
                        "对象与指定类型不兼容", "提示！");
            }
        }   
    }
}

三、生成