i'；m有两个不同的运行时错误

当不止一次发生运行时错误时，以及当我想删除我在文本框中写的内容时，我首先会得到两个运行时错误（｛"Sequence contains than one element"｝=>System.InvalidOperationException）我该怎么办？

这是我的代码

 private AutoCompleteStringCollection GetLinqDataSourceForString(string p)
    {
        var c = new AutoCompleteStringCollection();
        c.Add( Getauto(p));
        return c;
    }
   private void txtSearch_TextChanged(object sender, EventArgs e)
    {
        txtSearch.AutoCompleteSource = AutoCompleteSource.CustomSource;
        txtSearch.AutoCompleteMode = AutoCompleteMode.Suggest;
          txtSearch.AutoCompleteCustomSource = GetLinqDataSourceForString(txtSearch.Text);
      } 

这个在另一层从数据库获取信息

    public string AutoUpdate(string _search)
    {
        using (var context = new Phone_BookEntities1())
        {
            var c = (from d in context.Cantacts
                     where d.Cantact1.StartsWith(_search)
                     select d.Cantact1).SingleOrDefault();
            return c;
        }
    }