I encountered the following issue and couldn't find any information on the internet on this issue:

The null value cannot be assigned to a member with type System.Guid which is a non-nullable value type.

After quite a struggle I solved it as follow (which really is bizarre and still unexplained):

 I change:

SELECT *, guidanceType.name as guidanceType, legacyID as refid
FROM  HospitalGuide INNER JOIN
               GuidanceType ON HospitalGuide.GuidanceTypeId = GuidanceType.Id

 To:

SELECT HospitalGuide.*, guidanceType.name as guidanceType, legacyID as refid
FROM  HospitalGuide INNER JOIN
               GuidanceType ON HospitalGuide.GuidanceTypeId = GuidanceType.Id

 and it solved my problem.

add the following attributes:

OnClientClick="if(!confirm('Are you sure?'))return false;"

to your button / link button etc. e.g.

<asp:LinkButton ID="ButtonReset" runat="server" Text="Reset"

onclick="ButtonReset_Click" OnClientClick="if(!confirm('Are you sure?'))return false;" />

The easiest work around I guess is to do a javascript return false on trying to change the value, thus:

onclick="return false;"

Here are the ASP.Net and HTML versions:

<asp:CheckBox ID="Whatever" runat="server" onclick="return false;" />

<input type="checkbox" onclick="return false;" />