title: Add distinct items from a list to another list
body: <p>I would like to accomplish what the title states but I don't know how to go about doing so. </p>
  
  <p>I have 2 lists: </p>
  
  <pre><code>public List&lt;int[,]&gt; LongList = new List&lt;int[,]&gt;();
  public List&lt;int[,]&gt; UniqueList = new List&lt;int[,]&gt;();
  </code></pre>
  
  <p>To further explain, here's a scenario:</p>
  
  <p>Puzzles:</p>
  
  <pre><code>public int[,] puzzle1 = new int [3,3] { {1,2,3},
                                              {8,4,0},
                                              {7,6,5} }; //[1,2,3;8,4,0;7,6,5]
  
      public int[,] puzzle2 = new int [3,3] { {8,7,6},
                                              {1,0,5},
                                              {2,3,4}  }; //[8,7,6;1,0,5;2,3,4]
  
  
      public int[,] puzzle3 = new int [3,3] { {7,6,3},
                                              {1,0,2},  
                                              {8,4,5}  }; //[7,6,3;1,0,2;8,4,5]
  </code></pre>
  
  <p>LongList contains:</p>
  
  <pre><code>LongList.Add(puzzle1); 
  LongList.Add(puzzle1); 
  LongList.Add(puzzle1); 
  LongList.Add(puzzle1);
  LongList.Add(puzzle2);
  LongList.Add(puzzle2);
  LongList.Add(puzzle3);
  LongList.Add(puzzle3);
  LongList.Add(puzzle3);
  </code></pre>
  
  <p>I would like Unique list to hold the UNIQUE values from LongList.
  AS IF this happened:</p>
  
  <pre><code>UniqueList.Add(puzzle1);
  UniqueList.Add(puzzle2);
  UniqueList.Add(puzzle3);
  </code></pre>
  
  <p>As an equation: UniqueList = Distinct values from LongList</p>
  
  <p>List is full of multiple reoccurring values &amp; I would like to take only the unique ones and put them into <code>UniqueList</code>.</p>
  
  <p>I'm trying to complete a puzzle and the <code>LongList</code> will contain multiple references of the same same puzzle and more. To make it simple for case of discussion:</p>
  
  <p><code>LongList</code> values: 1,1,1,1,2,2,3,4,4,4,4,5,5</p>
  
  <p>I would like <code>UniqueList</code> to contain the puzzles: 1,2,3,4,5</p>