Using closedXML C# library, How can I figure out what data is causing this error when trying to save

Question 1

Since you have invalid characters in the data / strings you put into the ClosedXML sheet, you have to find them and get them out.

The simplest solution is to add

.Replace((0x0B).ToString(), " ")

to all your strings to get rid of the vertical tabs and replace them with spaces.

Question 2

Since ClosedXML is an open source project, the simplest way of tracking the error down would be building it from the source^*, and then running your code against the library in debug mode.

Once you see the full stack trace, you should be able to identify the spot from which the error is coming. Good chances are that it is a bug in the way the ClosedXML project uses Microsoft XML libraries, because the error that you mentioned is reported by a library outside the ClosedXML project.

^* I downloaded the project, and tried building it. Everything in the closedxml-79843.zip package builds correctly.

Question 3

Since ClosedXML doesn't prevent you from using the 0x0B character in values, you'll either have to scrub your data of it yourself (as suggested by @Raidri), or you could force and exception, or do a string replace when the value is set. I've created a sample program below which uses Castle's Dynamic Proxy to wrap the IXLWorksheet and IXLCell interfaces. Firstly, we proxy the IXLWorksheet values (which returned from adding a new worksheet as in the example below, or by indexing an existing worksheet). This needs to be done manually via a method call; everything else from then on is set up. When accessing cells (via the Cell methods, or the ActiveCell property) a proxied IXLCell value is returned which checks the data being set via the Value property and the SetValue method. The check is done in the ValidateMethodInterceptor as per the comments. This whole mechanism can be left in your codebase and turned on/off via a switch in the Program.Proxy method if you so desire.

As a further alternative, the package EPPlus (which has similar functionality to ClosedXML) doesn't crash when confronted with the VT character. Instead it replaces it with the value _x00B_. Perhaps a switch would be more beneficial?

internal class Program
{
    private static void Main(string[] args)
    {
        var stream = new MemoryStream();

        using (stream)
        {
            using (var workbook = new XLWorkbook())
            {
                using (var worksheet = Proxy(workbook.Worksheets.Add("Sheet 1")))
                {
                    worksheet.Cell("A1").Value = "This  is a test";
                    worksheet.Cell("A2").Value = "This \v is a test";
                    workbook.SaveAs(stream);
                }
            }
        }
    }

    public static IXLWorksheet Proxy(IXLWorksheet target)
    {
        var generator = new ProxyGenerator();
        var options = new ProxyGenerationOptions { Selector = new WorksheetInterceptorSelector() };

        return generator.CreateInterfaceProxyWithTarget<IXLWorksheet>(target, options);
    }
}

public class WorksheetInterceptorSelector : IInterceptorSelector
{
    private static readonly MethodInfo[] methodsToAdjust;
    private readonly ProxyCellInterceptor proxyCellInterceptor = new ProxyCellInterceptor();

    static WorksheetInterceptorSelector()
    {
        methodsToAdjust = typeof(IXLWorksheet).GetMethods()
            .Where(x => x.Name == "Cell")
            .Union(new[] { typeof(IXLWorksheet).GetProperty("ActiveCell").GetGetMethod() })
            .ToArray();
    }

    #region IInterceptorSelector Members

    public IInterceptor[] SelectInterceptors(System.Type type, System.Reflection.MethodInfo method, IInterceptor[] interceptors)
    {
        if (!methodsToAdjust.Contains(method))
            return interceptors;

        return new IInterceptor[] { proxyCellInterceptor }.Union(interceptors).ToArray();
    }

    #endregion
}

public class CellInterceptorSelector : IInterceptorSelector
{
    private static readonly MethodInfo[] methodsToAdjust = new[] { typeof(IXLCell).GetMethod("SetValue"), typeof(IXLCell).GetProperty("Value").GetSetMethod() };

    private ValidateMethodInterceptor proxyCellInterceptor = new ValidateMethodInterceptor();

    #region IInterceptorSelector Members

    public IInterceptor[] SelectInterceptors(System.Type type, MethodInfo method, IInterceptor[] interceptors)
    {
        if (method.IsGenericMethod && method.Name == "SetValue" || methodsToAdjust.Contains(method))
            return new IInterceptor[] { proxyCellInterceptor }.Union(interceptors).ToArray();
        return interceptors;
    }

    #endregion
}

public class ProxyCellInterceptor : IInterceptor
{
    #region IInterceptor Members

    public void Intercept(IInvocation invocation)
    {
        invocation.Proceed();

        //Wrap the return value
        invocation.ReturnValue = Proxy((IXLCell)invocation.ReturnValue);
    }

    #endregion

    public IXLCell Proxy(IXLCell target)
    {
        var generator = new ProxyGenerator();
        var options = new ProxyGenerationOptions { Selector = new CellInterceptorSelector() };

        return generator.CreateInterfaceProxyWithTarget<IXLCell>(target, options);
    }
}

public class ValidateMethodInterceptor : IInterceptor
{
    #region IInterceptor Members

    public void Intercept(IInvocation invocation)
    {
        var value = invocation.Arguments[0];

        //Validate the data as it is being set
        if (value != null && value.ToString().Contains('\v'))
        {
            throw new ArgumentException("Value cannot contain vertical tabs!");
        }

        //Alternatively, you could do a string replace:
        //if (value != null && value.ToString().Contains('\v'))
        //{
        //    invocation.Arguments[0] = value.ToString().Replace("\v", Environment.NewLine);
        //}

        invocation.Proceed();
    }

    #endregion
}