//C#通过ToUpper()方法将字符串转换成大写，代码如下：
string sentence= "this is in upper case.";
Console.WriteLine(sentence.ToUpper());

//C#通过ToLower()方法将字符串转换成小写，代码如下
string sentence= "this is in Lower case.";
Console.WriteLine(sentence.ToLower());

//c#无视大小写对比字符串
//方法1.
string s1 = "abcd";  
string s2 = "ABCD";  
 if(s1.ToUpper()==s2.ToUpper())  
{  
    Console.WriteLine("在忽略大小写的情况下比较，这两个字符串相同");  
}  
//方法2.
if(s1.Equals(s2,StringComparison.OrdinalIgnoreCase))  
 {  
     Console.WriteLine("在忽略大小写的情况下比较，这两个字符串相同");  

 }