This error might occur if you are using website and you have the report class in the website directly. This is due to the fact that when using website, after rebuild the following would always be different and it does not match with the originally assigned report: App_Code.unch8s_n. That is why we recommend following our best practices and have the report in a separate class library that is referenced in the web app/site.

This exception might surface if you try to use objects which do not implement ISerializable for Report Datasource. For example if this is IList, you can try using List instead as we show in our cars example. You can also use the NeedDataSource event of the report and assign the datasource to the "processing report", thus avoiding any need for serialization/deserialization. Another approach is to move on to DataSet or DataTable:

CopyC#

DataTable dt = new DataTable();
DataColumn col = new DataColumn("ColumnName", typeof(string));
//....
dt.Columns.Add(col);
foreach (object Item in MyIList)
{
    DataRow row = dt.NewRow();
    row["ColumnName"] = Item;
    //.....
    dt.Rows.Add(row);
}

During the installation of Telerik Reporting on dev machine, Telerik assemblies are added to GAC. When deploying a project using Visual Studio's built-in functionality, the assemblies from the GAC are not copied automatically, so you need to make sure the assemblies physically exist in the bin folder of your applicatio. Full details are available in the Deploying Applications using Telerik Reporting section.

There are three cases in which this error message occurs:

• The function is typed in manually without rebuilding the class library, thus it does not yet exist.
• Using the function with wrong number/type of parameters. The passed fields must match the function signature you've defined originally.
• The field specified in the function is null. Make sure such cases are handled in the user function.

1. Check if runat="server" is present in your web page's head tag.

2. Check if the web report viewer's http handler is registered in the web.config file. If the app is deployed on IIS7 and setup to work in integrated mode, make sure preCondition attribute is present at the end of Telerik.ReportViewer handler:

   CopyXML

   <add 
       name="Telerik.ReportViewer.axd_*" 
       type="Telerik.ReportViewer.WebForms.HttpHandler, Telerik.ReportViewer.WebForms, Version=x.x.x.x, Culture=neutral, PublicKeyToken=a9d7983dfcc261be" 
       path="Telerik.ReportViewer.axd" verb="*" 
       preCondition="integratedMode"
   />

3. Check if the web report viewer's styles are registered on the page. This can be accomplished with http debugging proxy like Fiddler.
4. If above steps are correct and problem persists, check for defined global styles on page that might mess up the viewer's styles. Remove any style declarations from page to make sure this is the culprit. If proves so, make sure global styles do not affect html elements directly, but are applied through the CssClass property.