Why is this extension method not working?

public static string ToTrimmedString(this DataRow row, string columnName)
{
    return row[columnName].ToString().Trim();
}

Compiles fine but it doesn't show up in intellisense of the DataRow...

My guess is you haven't included the namespace.

Ensure this method is in a static class of its own, separate class from the consuming DataRow.

namespace MyProject.Extensions
{
   public static class DataRowExtensions
   {
      //your extension methods
   }
}

In your consumer, ensure you're:

using MyProject.Extensions

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