目录

一、关于ToUpper()和ToLower()

1.ToUpper()

2.ToLower()     

3.小结

二、实例 

三、生成效果

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一、关于ToUpper()和ToLower()

1.ToUpper()

        使用字符串对象的ToUpper方法可以将字符串中的字母全部转换为大写。

string P_str_book ="mingribook".ToUpper();

2.ToLower()     

        使用字符串对象的ToLower方法可以将字符串中的字母全部转换为小写。

string P_str_book ="MINGRIBOOK".ToLower();

        字符串在创建后就成为不可变的对象，当调用字符串对象的方法操作字符串时，会产生新的字符串对象，而不是更改原来的字符串对象。

3.小结

        在深入使用字符串之前，有一个概念一定要理解，字符串是不可变的对象。理解了这一概念，对后面熟练使用字符串有着很大的帮助。字符串的不可变性，意味着每当对字符串进行操作时，都将产生一个新的字符串对象，如果频繁地操作字符串对象，会在托管堆中产生大量的无用字符串，增加垃圾收集器的压力，从而造成系统资源的浪费。

二、实例 

// 将字母全部转换为大写或小写
namespace _035
{public partial class Form1 : Form{private GroupBox? groupBox1;private TextBox? textBox2;private RadioButton? radioButton2;private RadioButton? radioButton1;private Button? button1;private TextBox? textBox1;public Form1(){InitializeComponent();Load += Form1_Load;}private void Form1_Load(object? sender, EventArgs e){// // textBox2// textBox2 = new TextBox{Location = new Point(49, 101),Name = "textBox2",Size = new Size(189, 23),TabIndex = 4};// // radioButton2// radioButton2 = new RadioButton{AutoSize = true,Location = new Point(188, 64),Name = "radioButton2",Size = new Size(50, 21),TabIndex = 3,TabStop = true,Text = "小写",UseVisualStyleBackColor = true};// // radioButton1// radioButton1 = new RadioButton{AutoSize = true,Location = new Point(130, 65),Name = "radioButton1",Size = new Size(50, 21),TabIndex = 2,TabStop = true,Text = "大写",UseVisualStyleBackColor = true};// // button1// button1 = new Button{Location = new Point(49, 63),Name = "button1",Size = new Size(75, 23),TabIndex = 1,Text = "转换",UseVisualStyleBackColor = true};button1.Click += Button1_Click;// // textBox1// textBox1 = new TextBox{Location = new Point(49, 22),Name = "textBox1",Size = new Size(189, 23),TabIndex = 0,Text = "请输入字符串",TextAlign = HorizontalAlignment.Center};textBox1.MouseClick += TextBox1_MouseClick;// // groupBox1// groupBox1 = new GroupBox{Dock = DockStyle.Fill,Location = new Point(0, 0),Name = "groupBox1",Size = new Size(289, 136),TabIndex = 0,TabStop = false,Text = "大小写转换"};groupBox1.Controls.Add(textBox2);groupBox1.Controls.Add(radioButton2);groupBox1.Controls.Add(radioButton1);groupBox1.Controls.Add(button1);groupBox1.Controls.Add(textBox1);groupBox1.SuspendLayout();// // Form1// AutoScaleDimensions = new SizeF(7F, 17F);AutoScaleMode = AutoScaleMode.Font;ClientSize = new Size(289, 136);Controls.Add(groupBox1);Name = "Form1";StartPosition = FormStartPosition.CenterScreen;Text = "字母大小写转换";}private void Button1_Click(object? sender, EventArgs e){if (radioButton1!.Checked)  {textBox2!.Text = textBox1!.Text.ToUpper();  //将字符串转换为大写}else{textBox2!.Text = textBox1!.Text.ToLower();  //将字符串转换为小写}}private void TextBox1_MouseClick(object? sender, MouseEventArgs e){if (textBox1!.Text == "请输入字符串"){textBox1!.Text = string.Empty;}}}
}

三、生成效果