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