Top C# Interview Questions

What are the key features of C#?

What is the difference between 'continue' and 'break' statements in C#?

What is the purpose of the 'using' directive in C#?

What is the difference between 'struct' and 'class' in C#?

What is the purpose of the 'finally' block in a try-catch-finally statement?

What is the difference between 'override' and 'new' keywords in C#?

What is the purpose of the 'sealed' keyword in C#?

What is the difference between 'interface' and 'abstract class' in C#?

What is the purpose of the 'async' and 'await' keywords in C#?

What is the difference between 'Thread' and 'Task' in C#?

What is the purpose of the 'lock' statement in C#?

What is the difference between 'Dictionary' and 'Hashtable' in C#?

What is the purpose of the 'yield' keyword in C#?

What is the difference between 'IEnumerable' and 'IQueryable' in C#?

What is the purpose of the 'Lazy' class in C#?

What is the difference between 'string' and 'StringBuilder' in C#?

What is the purpose of the 'Regex' class in C#?

What is the difference between 'Debug' and 'Release' configurations in C#?

What is the purpose of the 'App.config' file in C#?

What is the difference between 'WebForms' and 'MVC' in C#?

What is the purpose of the 'Entity Framework' in C#?

What is the difference between 'ADO.NET' and 'Entity Framework' in C#?

Write a C# program to find the maximum value in an array.

public class Program {
  public static void Main() {
    
    int[] array = {
      1, 2, 3, 4, 5 }
      ;
      int max = array[0];
      for (int i = 1;
      i < array.Length;
      i++) {
        
        if (array[i] > max) {
          
          max = array[i];
        }
      }
      Console.WriteLine(max);
    }
  }

Write a C# program to reverse a string.

public class Program {
  public static void Main() {
    
    string str = "hello";
    char[] arr = str.ToCharArray();
    Array.Reverse(arr);
    string reversed = new string(arr);
    Console.WriteLine(reversed);
  }
}

Write a C# program to find the first duplicate in an array.

public class Program {
  public static void Main() {
    
    int[] array = {
      1, 2, 3, 2, 4 }
      ;
      Dictionary<int, bool> dict = new Dictionary<int, bool>();
      for (int i = 0;
      i < array.Length;
      i++) {
        
        if (dict.ContainsKey(array[i])) {
          
          Console.WriteLine(array[i]);
          break;
        }
        else {
          dict.Add(array[i], true);
        }
      }
    }
  }

Write a C# program to find the missing number in an array of consecutive integers.

public class Program {
  public static void Main() {
    
    int[] array = {
      1, 2, 3, 5 }
      ;
      int sum = (array.Length + 1) * (array.Length + 2) / 2;
      int actualSum = 0;
      for (int i = 0;
      i < array.Length;
      i++) {
        
        actualSum += array[i];
      }
      Console.WriteLine(sum - actualSum);
    }
  }

Write a C# program to find the maximum sum of a subarray.

public class Program {
  public static void Main() {
    
    int[] array = {
      -2, 1, -3, 4, -1, 2, 1, -5, 4 }
      ;
      int max = array[0];
      int current = array[0];
      for (int i = 1;
      i < array.Length;
      i++) {
        
        current = Math.Max(array[i], current + array[i]);
        max = Math.Max(max, current);
      }
      Console.WriteLine(max);
    }
  }

Write a C# program to find the first pair of elements that add up to a given sum.

public class Program {
  public static void Main() {
    
    int[] array = {
      1, 2, 3, 4, 5 }
      ;
      int sum = 7;
      Dictionary<int, bool> dict = new Dictionary<int, bool>();
      for (int i = 0;
      i < array.Length;
      i++) {
        
        if (dict.ContainsKey(sum - array[i])) {
          
          Console.WriteLine(array[i] + " " + (sum - array[i]));
          break;
        }
        else {
          dict.Add(array[i], true);
        }
      }
    }
  }

Write a C# program to find the minimum window that contains all elements of a given array.

public class Program {
  public static void Main() {
    
    string str = "abc";
    string pattern = "abc";
    Dictionary<char, int> dict = new Dictionary<char, int>();
    foreach (char c in pattern) {
      
      if (dict.ContainsKey(c)) {
        
        dict[c]++;
      }
      else {
        dict.Add(c, 1);
      }
    }
    int required = dict.Count;
    int left = 0;
    int minLen = int.MaxValue;
    int formed = 0;
    Dictionary<char, int> windowCounts = new Dictionary<char, int>();
    for (int right = 0;
    right < str.Length;
    right++) {
      
      char c = str[right];
      if (dict.ContainsKey(c)) {
        
        if (windowCounts.ContainsKey(c)) {
          
          windowCounts[c]++;
        }
        else {
          windowCounts.Add(c, 1);
        }
        if (windowCounts[c] == dict[c]) {
          
          formed++;
        }
      }
      while (left <= right && formed == required) {
        
        c = str[left];
        if (right - left + 1 < minLen) {
          
          minLen = right - left + 1;
        }
        if (dict.ContainsKey(c)) {
          
          windowCounts[c]--;
          if (windowCounts[c] < dict[c]) {
            
            formed--;
          }
        }
        left++;
      }
    }
    Console.WriteLine(minLen);
  }
}

Write a C# program to find the maximum length of a subarray with a given sum.

public class Program {
  public static void Main() {
    
    int[] array = {
      1, 2, 3, 4, 5 }
      ;
      int sum = 5;
      int maxLen = 0;
      for (int i = 0;
      i < array.Length;
      i++) {
        
        int currentSum = 0;
        for (int j = i;
        j < array.Length;
        j++) {
          
          currentSum += array[j];
          if (currentSum == sum) {
            
            maxLen = Math.Max(maxLen, j - i + 1);
          }
        }
      }
      Console.WriteLine(maxLen);
    }
  }

Write a C# program to find the minimum number of operations to convert one string to another.

public class Program {
  public static void Main() {
    
    string str1 = "kitten";
    string str2 = "sitting";
    int[,] dp = new int[str1.Length + 1, str2.Length + 1];
    for (int i = 0;
    i <= str1.Length;
    i++) {
      
      for (int j = 0;
      j <= str2.Length;
      j++) {
        
        if (i == 0) {
          
          dp[i, j] = j;
        }
        else if (j == 0) {
          
          dp[i, j] = i;
        }
        else if (str1[i - 1] == str2[j - 1]) {
          
          dp[i, j] = dp[i - 1, j - 1];
        }
        else {
          dp[i, j] = 1 + Math.Min(Math.Min(dp[i - 1, j], dp[i, j - 1]), dp[i - 1, j - 1]);
        }
      }
    }
    Console.WriteLine(dp[str1.Length, str2.Length]);
  }
}