title: Dictionary if Key exist append if not add new element C#
body: <p>I am having </p>
  
  <pre><code>Dictionary&lt;String, List&lt;String&gt;&gt; filters = new Dictionary&lt;String, List&lt;String&gt;&gt;();
  </code></pre>
  
  <p>which is having values like <code>country = us</code>. till now I am able to add it when key is not repeated. now when key <code>country</code> is repeated. it is showing that the key is already present. </p>
  
  <p>what I want is How to add multiple values in the same key. I am not able to do it. Please suggest something.</p>
  
  <pre><code>for (int i = 0; i &lt; msgProperty.Value.Count; i++)
  {
      FilterValue.Add(msgProperty.Value[i].filterValue.Value);
      filterColumn = msgProperty.Value[i].filterColumnName.Value;
      filters.Add(filterColumn, FilterValue);
  }
  </code></pre>
  
  <p>what I want </p>
  
  <blockquote>
    <p>country = US,UK</p>
  </blockquote>