"Collection was modified; enumeration operation may not execute "

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C#3.0

C#

"Collection was modified; enumeration operation may not execute " Why this error is coming when execute my below code:

This is my Window Form Application in .Net

1.I am using a Global class for reading instance message and shown it to a Textbox
2.It is executing fine but some time showing error like "Collection was modified; enumeration operation may not execute " in second foreach looping like

foreach (var values in dict[item])
                   {

3. My global Class is in which i am storing value in Dictionary object and retrived from that .

public static class GlobalVariables  
    {  
            
       public static Dictionary<string,>> dictionary = new Dictionary<string,>>();
                
        public static void SetMessage(String DBName, String Message)
        {                       
                  

            if (dictionary.ContainsKey(DBName))
            {                
                //dictionary.Remove(DBName);
                List<string> list = new List<string>();
                list.Clear();
                list.Add(Message);
                dictionary[DBName]=list;          
                   
            }
            else
            {
                
                    List<string> list = new List<string>();
                    list.Add(DateTime.Now.ToString());
                    dictionary.Add("Process Started",list);
                

            }          
                       
            
        }   
                           
    }

4.Here we can show the message in textbox from Dictionary object and error shown in the second foreach line like

foreach (var values in dict[item])
                    {

public void setTextBoxValue()
      {
          //Thread.Sleep(1000);
          Dictionary<string, List<string>> dict = GlobalVariables.dictionary;
            //string strMessage = Convert.ToString(GlobalVariables.message);

            if (dict.Keys.Count > 0)
            {
                builder.Remove(0, builder.Length);

                foreach (var item in dict.Keys)
                {

                    foreach (var values in dict[item])
                    {

                       string message = item + ":" + values.ToString();

                       builder.AppendLine(message);

                       builder.Append("\n\n");

                      textBox1.Text = builder.ToString();


                    }


                }
                Application.DoEvents();

            }


        }