How to throw an exception if data for a merge field does not exist?

Syncfusion Essential DocIO is a .NET Word library used to create, read, edit, and convert Word documents programmatically without Microsoft Word or interop dependencies. Using this library, you can throw an exception if a data for a merge field does not exist for mail merge in Word document using C#.

Steps to throw an exception if data for a merge field does not exist for mail merge:

1. Create a new .NET Core console application project.
2. Install the Syncfusion.DocIO.Net.Core NuGet package as a reference to your project from NuGet.org.
   
   

3. Include the following namespaces in Program.cs file.
   C#

using Syncfusion.DocIO;
using Syncfusion.DocIO.DLS;

4. Use the following code example to perform a mail merge.
   C#

using (FileStream fileStream = new FileStream(Path.GetFullPath(@"Data/Template.docx"), FileMode.Open, FileAccess.ReadWrite))
{
   //Opens the template document.
   using (WordDocument document = new WordDocument(fileStream, FormatType.Docx))
   {
       //Sets “ClearFields” to true to remove empty mail merge fields from document.
       document.MailMerge.ClearFields = false;
       //Uses the mail merge event to throw exception if the field is not in the data source.
       document.MailMerge.BeforeClearField += new BeforeClearFieldEventHandler(BeforeClearFieldEvent);
       //Execute mail merge.
       document.MailMerge.ExecuteGroup(GetDataTable());
       //Creates file stream.
       using (FileStream outputStream = new FileStream(Path.GetFullPath(@"Output/Result.docx"), FileMode.Create, FileAccess.ReadWrite))
       {
           //Saves the Word document to file stream.
           document.Save(outputStream, FormatType.Docx);
       }
   }
} 

5. Use the following helper method to throw an exception if a mail merge field isn’t found in the data source.
   C#

private static void BeforeClearFieldEvent(object sender, BeforeClearFieldEventArgs args)
{
    try
    {
        // Check if the field is not present in the data source.
        if (!args.HasMappedFieldInDataSource)
        {
            // Throw an exception if the field is missing in the data source.
            throw new Exception($"The field {args.FieldName} is not defined in data source");
        }
    }
    // Output the exception message to the console.
    catch (Exception ex) { Console.WriteLine(ex.Message); }
} 

6. Use the following helper method to create a data table to perform mail merge.
   C#

private static DataTable GetDataTable()
{
   //Create an instance of DataTable.
   DataTable dataTable = new DataTable("Employee");
   //Add columns.
   dataTable.Columns.Add("EmployeeId");
   dataTable.Columns.Add("City");
   //Add records to the DataTable.
   // Create a new row for EmployeeId "1001" and set the "City" to null.
   DataRow row;
   row = dataTable.NewRow();
   row["EmployeeId"] = "1001";
   row["City"] = null;
   dataTable.Rows.Add(row);
   // Create a new row for EmployeeId "1002" and set the "City" to an empty string.
   row = dataTable.NewRow();
   row["EmployeeId"] = "1002";
   row["City"] = "";
   dataTable.Rows.Add(row);
   // Create a new row for EmployeeId "1003" and set the "City" to "London".
   row = dataTable.NewRow();
   row["EmployeeId"] = "1003";
   row["City"] = "London";
   dataTable.Rows.Add(row);

   return dataTable;
} 

You can download a complete working sample that throw an exception if data for a merge field does not exist for mail merge from the GitHub.
Take a moment to peruse the documentation where you can find basic Word document processing options along with the features like mail merge, merge, split, and compare Word documents, find and replace text in the Word document, protect the Word documents, and most importantly, the PDF and Image conversions with code examples.

Conclusion
I hope you enjoyed learning about how to throw an exception if a field doesn’t exist in the data source.

You can refer to our ASP.NET Core DocIO feature tour page to know about its other groundbreaking feature representations and documentation, and how to quickly get started for configuration specifications. You can also explore our ASP.NET Core DocIO example to understand how to create and manipulate data.

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