class Program
    {
        static void Main(string[] args)
        {
            int start = 0;
           int[] numbers = new int[7] { 2,1,2,1,5,6,5};
    }

int[] numbers = new [] { 2, 1, 2, 1, 5, 6, 5 };
  int toFind = 5;

  // all indexes of "5" {4, 6}
  int[] indexes = numbers
    .Select((v, i) => new {
      value = v,
      index = i
    })
    .Where(pair => pair.value == toFind)
    .Select(pair => pair.index)
    .ToArray();

List<int> indexes = new List<int>();
for (int i = 0; i < numbers.Length; i++)
{
    if (numbers[i] == yourNumber)
        indexes.Add(i);
}

// The value you are looking for it's indices
var value = 2;

// A list in which you will store the indices.
var indices = new List<int>();

// Loop through the elements of your array
for(i=0; i<numbers.Length; i++)
{
    // Check if the current value is equal to the value you are looking for.
    if(numbers[i]==value)
    {
        // If so, then add it's index in the list of indices.
        indices.Add(i);
    }
}

var indices = numbers.Select((number, index) => new { number, index })
                     .Where(x => x.number == value)
                     .Select(x => x.index);

var indexes = numbers
  .Select((x, idx) => new { x, idx })
  .Where(c => c.x == number)
  .Select(c => c.idx);