How to Check for the Existence of a Dictionary Key in C#

1. Use try-catch to Check for the Existence of a Dictionary Key in C#
2. Use ContainsKey() to Check for the Existence of a Dictionary Key in C#
3. Use TryGetValue() to Check for the Existence of a Dictionary Key in C#

A Dictionary tends to map keys and values. It contains a specific key to which a particular value is mapped. Duplicate keys are not allowed, and that is the entire goal of a Dictionary.

Today we will be looking at how we can check if a key already exists in a Dictionary or not.

Use try-catch to Check for the Existence of a Dictionary Key in C#

Let’s first start with creating a dictionary called students that maps each specific student ID to the CGPA:

Dictionary<int, double> students = new Dictionary<int, double>();
students[101] = 3.22;
students[102] = 3.44;
students[103] = 2.98;

Because the CGPAs are supposed to be in double format, we have defined the <TKEY, TVALUE> as <int, double> respectively for IDs and CGPAs.

Let’s say we want to find if there exists an entry for the student 102 in students. It does, but we need a code to tell us straight.

So to do that, we can use a try-catch exception as follows:

try {
  Console.WriteLine(students[102].ToString());
} catch (Exception e) {
  Console.WriteLine("No such key present");
}

OUTPUT:

3.44

But if we now try to do the same thing for student 104, the following happens:

OUTPUT:

No such key present

Why? Well, the ToString() method only works for values that are not null. Because student 104 is null and has no entry in students, an exception occurs.

And rather than throwing the exception, we tend to catch it and present the message No such key present in the console.

So it is an effective way to check whether a key is present. But can we make it simpler?

Use ContainsKey() to Check for the Existence of a Dictionary Key in C#

Observe the following code:

if (!students.ContainsKey(104)) {
  Console.WriteLine("No such key present");
}

This will present an error because no such key is present.

OUTPUT:

No such key present

Use TryGetValue() to Check for the Existence of a Dictionary Key in C#

If we do the following:

double getval = 0.0;
if (!students.TryGetValue(104, out getval)) {
  Console.WriteLine("No such key present");
}

This tends to return the output as follows:

No such key present

The TryGetValue() tends to obtain a value for a key if it exists. The value is obtained in the second parameter.

Hence it is sent with the out keyword. And the variable to get this value is declared before the scope, which is getval.

The full code for all of these different methods has been included below:

using System;
using System.Collections.Generic;
using System.Text;

namespace jinlku_console {
  class coder {
    static void Main(String[] args) {
      Dictionary<int, double> students = new Dictionary<int, double>();
      students[101] = 3.22;
      students[102] = 3.44;
      students[103] = 2.98;

      // TRY CATCH BLOCK
      try {
        Console.WriteLine(students[104].ToString());
      } catch (Exception e) {
        Console.WriteLine("No such key present");
      }

      // CONTAINS KEY METHOD
      if (!students.ContainsKey(104)) {
        Console.WriteLine("No such key present");
      }

      // TRYGETVALUE

      double getval = 0.0;
      if (!students.TryGetValue(104, out getval)) {
        Console.WriteLine("No such key present");
      }
    }
  }
}