How do I fix the error "Expression must evaluate to a node-set" setting web.config entries?

Question

I'm trying to update web.config in SharePoint when the feature is activated. I have created Event receiver. The code inside receiver is:

    public override void FeatureActivated(SPFeatureReceiverProperties properties)
    {
        SPWebApplication webApp = (properties.Feature.Parent as SPSite).WebApplication;

        SPWebConfigModification modification = new SPWebConfigModification("mode", "system.web/customErrors");
        modification.Owner = "modifyAccountFeatureOwner";
        modification.Sequence = 0;
        modification.Type = SPWebConfigModification.SPWebConfigModificationType.EnsureAttribute;
        modification.Value = "Off";         
        webApp.WebConfigModifications.Add(modification);

        webApp.Update();            
        webApp.Farm.Services.GetValue<SPWebService>().ApplyWebConfigModifications(); 
    }

When I'm deploying feature I have received the following error: "Error occurred in deployment step 'Activate Features': Expression must evaluate to a node-set."

What is wrong with this?

Answer 1

I think the error occurs on this line:

SPWebConfigModification modification = new SPWebConfigModification("mode", "system.web/customErrors");

And the problem is the second parameter

"system.web/customErrors"

which is an XPath expression. And this expression seems to be incorrect, which means it does not select an existing node in the web.config file. It probably has to be:

"configuration/system.web/customErrors/@mode"

Please note: I did not test this.

Answer 2

Add the following line after webApp.WebConfigModifications.Add(modification);

webApp.WebConfigModifications.Clear();

it clears all the non-required modifications from the object.