Question

I need to get some random colors to draw a pie. My code works but it can take the same color again and again

    Random r = new Random();

    for (int i = 0; i < 20; i++)
    {
        int min = 0;
        int max = 255;
        int rand1 = r.Next(min, max);
        int rand2 = r.Next(min, max);
        int rand3 = r.Next(min, max);
        Color myColor = Color.FromArgb(rand1, rand2, rand3);  
        //drawing the pie here
   }

How can I rework it so that it’s not going to pick the same color again.

Answer 1

You could put the random colors you generate in a container, then check if a similar color by a certain delta has already been inserted in the container.

This way you won’t risk to pick up a different color but very similar, like:
RGB(0, 0, 0) and RGB(10, 2, 3)

const int DELTA_PERCENT = 10;
List<Color> alreadyChoosenColors = new List<Color>();

// initialize the random generator
Random r = new Random();

for (int i = 0; i < 20; i++)
{
    bool chooseAnotherColor = true;      
    while ( chooseAnotherColor )
    {
       // create a random color by generating three random channels
       //
       int redColor = r.Next(0, 255);
       int greenColor = r.Next(0, 255);
       int blueColor = r.Next(0, 255);
       Color tmpColor = Color.FromArgb(redColor, greenColor, blueColor);  

       // check if a similar color has already been created
       //
       chooseAnotherColor = false;
       foreach (Color c in alreadyChoosenColors)
       {
          int delta = c.R * DELTA_PERCENT / 100;
          if ( c.R-delta <= tmpColor.R && tmpColor.R <= c.R+delta )
          {
             chooseAnotherColor = true;
             break;
          }

          delta = c.G * DELTA_PERCENT / 100;
          if ( c.G-delta <= tmpColor.G && tmpColor.G <= c.G+delta )
          {
             chooseAnotherColor = true;
             break;
          }

          delta = c.B * DELTA_PERCENT / 100;
          if ( c.B-delta <= tmpColor.B && tmpColor.B <= c.B+delta )
          {
             chooseAnotherColor = true;
             break;
          }
        }
    }

    alreadyChoosenColors.Add(tmpColor);
    // you can safely use the tmpColor here

   }