Gets or sets the culture information to be used to convert the numeric strings such as currencies and formatted numerals in the imported data to numbers. By default it is set to System.Globalization.CultureInfo.InvariantCulture. This property setting will take effect only if the ConvertStrings property is set to true. 

public System.Globalization.CultureInfo ConversionCulture { get; set; } 

Public Property ConversionCulture() As System.Globalization.CultureInfo

When the ConversionCulture property is set to null (C#) or Nothing (VB.NET), ExcelWriter will throw this exception.

By default, when importing data, ExcelWriter will not convert numeric strings to numbers. If the ConvertStrings property is set to true, ExcelWriter will parse the strings into a decimal type by using the culture information specified through the ConversionCulture property. However if ConvertStrings is set to false, the setting of the ConversionCulture property will not take any effect and the numeric string will not be converted.

If ConvertStrings is enabled but no culture is set for the ConversionCulture property, then ExcelWriter will use the invariant culture, System.Globalization.CultureInfo.InvariantCulture, to parse the numeric strings. 

      DataImportProperties importProps = wb.CreateDataImportProperties();

      //--- Get ConversionCulture
      System.Globalization.CultureInfo conversionCulture = importProps.ConversionCulture;

      //--- In order for the ConversionCulture setting to take effect, ConvertStrings has to be enabled first 
      importProps.ConvertStrings = true;

      //--- Example 1: Set ConversionCulture to the current thread's culture
      importProps.ConversionCulture = System.Threading.Thread.CurrentThread.CurrentCulture;

      //--- Example 2: Set ConversionCulture specifically to the "en-US" culture 
      importProps.ConversionCulture = new System.Globalization.CultureInfo("en-US");

     Dim importProps As DataImportProperties = wb.CreateDataImportProperties()

     '--- Get ConversionCulture
     Dim conversionCulture As System.Globalization.CultureInfo =  importProps.ConversionCulture

     '--- In order for the ConversionCulture setting to take effect, ConvertStrings has to be enabled first 
     importProps.ConvertStrings = True 

     '--- Example 1: Set ConversionCulture to the current thread's culture
     importProps.ConversionCulture = System.Threading.Thread.CurrentThread.CurrentCulture

     '--- Example 2: Set ConversionCulture specifically to the "en-US" culture
     importProps.ConversionCulture = New System.Globalization.CultureInfo("en-US")