How to fix 'Value cannot be null. Parameter name: Key' error in c#

user_10829384 0
Viewed 36088

I was using Dictionary in C# and wanted to display a Key inputted by the user and display that key and its corresponding value if that key is present in the dictionary. Also, continue reading the lines until there is no more input.

using System;
using System.Collections.Generic;
using System.IO;

class Solution 
{
    static void Main(String[] args) 
    {
        string number = Console.ReadLine();
        int n;
        Int32.TryParse(number, out n);

        var phoneBook = new Dictionary<string, string>();

        for(int i = 0; i < n; i++)
        {
            string name = Console.ReadLine();
            string phoneNumber = Console.ReadLine();
            phoneBook.Add(name, phoneNumber);
        }

        foreach (var pair in phoneBook)
        {
            string name = pair.Key;
            string phoneNumber = pair.Value;
        }


        string x = Console.ReadLine();
        if(!phoneBook.ContainsKey(x))
        {
            Console.WriteLine("Not Found");
        }
        else
        {
            string result = phoneBook[x];
            Console.Write(x);
            Console.Write("=");
            Console.Write(result);
        }
    }
}


Error message:

    Unhandled Exception:
    System.ArgumentNullException: Value cannot be null.
    Parameter name: key
      at System.Collections.Generic.Dictionary2[TKey,TValue].FindEntry (TKey key) <0x7fb28d7c9320 + 0x0023c> in <3833a6edf2074b959d3dab898627f0ac>:0 
      at System.Collections.Generic.Dictionary2[TKey,TValue].ContainsKey (TKey key) <0x7fb28d7c8cd0 + 0x00009> in <3833a6edf2074b959d3dab898627f0ac>:0 
      at Solution.Main (System.String[] args) [0x00096] in solution.cs:30 
    [ERROR] FATAL UNHANDLED EXCEPTION: System.ArgumentNullException: Value cannot be null.
    Parameter name: key
      at System.Collections.Generic.Dictionary2[TKey,TValue].FindEntry (TKey key) <0x7fb28d7c9320 + 0x0023c> in <3833a6edf2074b959d3dab898627f0ac>:0 
      at System.Collections.Generic.Dictionary2[TKey,TValue].ContainsKey (TKey key) <0x7fb28d7c8cd0 + 0x00009> in <3833a6edf2074b959d3dab898627f0ac>:0 
      at Solution.Main (System.String[] args) [0x00096] in solution.cs:30
c#
5 Answers

Change the line: 

if(!phoneBook.ContainsKey(x))

to:

if(x == null || !phoneBook.ContainsKey(x))

The error is pointing to that line of code, I believe. Please add a comment that points to the faulting line of code (to make it easier for us), and please reformat your exception text as code (with the back-ticks removed) so that the stack is readable.

user_47394880

The short answer is that one of your objects is null. Most likely one of the variables being set from a call to Console.ReadLine.

It is a common belief that Console.ReadLine cannot return null. This is false. It will return null if Control-Z is entered at the console.

As such, you need to check for null before interacting with the Dictionary (e.g. calls to Add or ContainsKey). Alternatively, change your ReadLine calls from:

string x = Console.ReadLine();

to:

string x = Console.ReadLine() ?? string.Empty;

to ensure the variable can't be null.

Also, as an optimisation, consider using TryGetValue vs ContainsKey to avoid the later [] call.

user_340920 
using System;
using System.Collections.Generic;
using System.IO;
class Solution 
{
    static void Main(String[] args) 
    {   
        var phoneBook = new Dictionary<string, string>();

        int n = Convert.ToInt32(Console.ReadLine());

        for(int i = 0; i < n; i++)
        {
            string[] record = Console.ReadLine().Split();
            string name = record[0];
            string phoneNumber = record[1];
            phoneBook.Add(name, phoneNumber); 
        }

        string x;
        while((x = Console.ReadLine()) != null)
        {
            if(phoneBook.ContainsKey(x))
            {
                Console.WriteLine(x + "=" + phoneBook[x]);
            }
            else
            {
                Console.WriteLine("Not found");
            }
        }
    }
}

This is the final solution which worked for me. Changes: 1. Added a while loop condition to avoid null search string. 2. Removed the foreach loop which was not needed. 3. In for loop for taking dictionary input used the array to store two strings in each record/entry of the dictionary.

user_108293840

I had this exception when trying to saveChanges my dbContext on ef core and asp.net core version 2.2. I was running my project on visual studio 2022. finally, I found out I should install dotnet-sdk-2.2.110-win-x64.exe and dotnet-runtime-2.2.8-win-x64.exe. and my problem was solved!

user_64079880

This error can sometimes be experienced because of the relationships of its related tables. To avoid this, typing [jsonignore] above the relation lines may solve your problem.

user_191154610
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