Summary 

After editing an Excel `.xlsx` file using Telerik RadSpreadsheet and saving it via the default Save option, the saved file becomes corrupted. It no longer opens in RadSpreadsheet (throws an error) and shows a repair warning in Microsoft Excel. 

Reproduction Steps 

1. Upload an Excel `.xlsx` file to the `ABC` folder on the server. 

2. Load the file in RadSpreadsheet via a basic viewer page. 

3. Make any small edit (e.g., change a cell’s value). 

4. Click the built-in Save option in the RadSpreadsheet toolbar. 

5. Attempt to: 

   - Reopen the saved file in RadSpreadsheet → Error: Object reference not set to an instance of an object. 

   - Open in Excel → Warning: “We found a problem with some content in ‘filename’. Do you want us to try to recover as much as we can?” 

Files Attached 

- `Original.xlsx` — Before editing, opens fine in both RadSpreadsheet and Excel. 

- `Modified.xlsx` — After saving via RadSpreadsheet, causes errors. 

- Screenshot of: 

   - RadSpreadsheet error : 

   - Excel repair prompt 

Code Snippet : 

<telerik:RadSpreadsheet ID="sample" runat="server" Visible="false" style="font-size: 10px;" /> 

protected void Page_Load(object sender, EventArgs e) 

{ 

    string fileName = (string)Session["SelectedFileName"]; 

    SheetLoad(fileName); 

} 

private void SheetLoad(string fileName) 

{ 

    try 

    { 

        string filePath = Server.MapPath("~/ABC/" + fileName); 

        if (!File.Exists(filePath)) 

        { 

            string errorMsg = "File not found: " + filePath; 

            ClientScript.RegisterStartupScript(this.GetType(), "alert", "alert('File not found!');", true); 

            return; 

        } 

        // Load spreadsheet using Telerik document provider 

        sample.Provider = new SpreadsheetDocumentProvider(filePath); 

        sample.Visible = true; 

    } 

    catch (Exception ex) 

    { 

        string errorMsg = "Error opening file: " + ex.Message + " | File: " + fileName; 

        ClientScript.RegisterStartupScript(this.GetType(), "alert", "alert('Error loading file!');", true); 

    } 

} 

**Observation:** 

* This only happens for **some files**, especially ones that likely contain advanced Excel features. 

* Other simpler files save and reload without any issue. 

**Assumption:** 

It seems the default save behavior of RadSpreadsheet is **not preserving some Excel structures**, leading to file corruption on save.