Problem to modify web.config in runtime

Question

I'm trying to modify the web.config file when the feature is activated with the code below:

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(); 
}

But I'm receiving this error in the step Activate Features:

Error occurred in deployment step 'Activate Features': '' is an invalid expression.

Anyone know what is causing this error?

Answer 1

try the below code.

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

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

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