populate a dropdownlist from an method declared as an IEnumerable in asp.net c#

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C#

ASP.NET

C#4.0

Hi how to populate an dropdownlist from an method declared as an IEnumerable?
Am getting an error: "Cannot implicitly convert type 'void' to 'string'"
on this line ddlUserType.DataValueField = ddlUserType.DataValueField = _UserTypeInfoList.Insert(0, new UserTypeInfo { UserType ="" }); from code behind file
This is the code am using from a class called UserTypeInfo:

public IEnumerable<UserTypeInfo> GetUserType()
    {
        using (SqlDataReader reader = SqlHelper.ExecuteReader(SqlHelper.GetConnection(), "[GetUserType]"))
        {
            if (reader.HasRows)
            {
                while (reader.Read())
                {
                    yield return new UserTypeInfo
                    {
                        UserTypeId = Convert.ToInt32(reader["UserTypeId"]),
                        UserType = Convert.ToString(reader["UserType"])
                      
                    };
                }
            }
        }

And this the codebehind that am using on PageLoad of a asp.x file:

if (!Page.IsPostBack)
        {

            UserTypeInfo _UserTypeInfo = new UserTypeInfo();
            List<UserTypeInfo> _UserTypeInfoList = _UserTypeInfo.GetUserType().ToList<UserTypeInfo>();
            ddlUserType.DataValueField = _UserTypeInfoList.Insert(0, new UserTypeInfo { UserType ="" });

        }

And Html code:

<asp:Label ID="lblUserType" runat="server" class="label">User Type :</asp:Label>
<asp:DropDownList ID="ddlUserType" runat="server"></asp:DropDownList>

Need help!!