C＃|检查StringDictionary是否包含特定键

StringDictionary.ContainsKey(String)方法用于检查StringDictionary是否包含特定键。

句法：

public virtual bool ContainsKey (string key);

在这里， key是要在StringDictionary中定位的键。

返回值：如果StringDictionary包含具有指定键的条目，则此方法返回true ，否则返回false 。

异常：如果键为null，则此方法将提供ArgumentNullException 。

下面的程序说明了StringDictionary.ContainsKey(String)方法的用法：

范例1：

// C# code to check if the
// StringDictionary contains
// a specific key
using System;
using System.Collections;
using System.Collections.Specialized;
  
class GFG {
  
    // Driver code
    public static void Main()
    {
  
        // Creating a StringDictionary named myDict
        StringDictionary myDict = new StringDictionary();
  
        // Adding key and value into the StringDictionary
        myDict.Add("G", "Geeks");
        myDict.Add("F", "For");
        myDict.Add("C", "C++");
        myDict.Add("DS", "Data Structures");
        myDict.Add("N", "Noida");
  
        // Checking if "DS" is contained in
        // StringDictionary myDict
        if (myDict.ContainsKey("DS"))
            Console.WriteLine("StringDictionary myDict contains the key");
        else
            Console.WriteLine("StringDictionary myDict does not contain the key");
    }
}

输出：

StringDictionary myDict contains the key

范例2：

// C# code to check if the
// StringDictionary contains
// a specific key
using System;
using System.Collections;
using System.Collections.Specialized;
  
class GFG {
  
    // Driver code
    public static void Main()
    {
  
        // Creating a StringDictionary named myDict
        StringDictionary myDict = new StringDictionary();
  
        // Adding key and value into the StringDictionary
        myDict.Add("G", "Geeks");
        myDict.Add("F", "For");
        myDict.Add("C", "C++");
        myDict.Add("DS", "Data Structures");
        myDict.Add("N", "Noida");
  
        // Checking if "null" is contained in
        // StringDictionary myDict
        // This should raise "ArgumentNullException"
        // as the key is null
        if (myDict.ContainsKey(null))
            Console.WriteLine("StringDictionary myDict contains the key");
        else
            Console.WriteLine("StringDictionary myDict does not contain the key");
    }
}

运行时错误：

Unhandled Exception:
System.ArgumentNullException: Value cannot be null.
Parameter name: key

笔记：

• 密钥以不区分大小写的方式处理，即在将密钥添加到字符串字典之前将其转换为小写形式。
• 此方法是O(1)操作。

参考：

• https://docs.microsoft.com/zh-cn/dotnet/api/system.collections.specialized.stringdictionary.containskey?view=netframework-4.7.2