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SaveAs() FileNotFoundException on .NET 10

Curtis Chau
Curtis Chau
Updated:

Calling WorkBook.SaveAs() in a .NET 10 project can make Visual Studio surface a FileNotFoundException for IronXL.XmlSerializers. In most cases this is a first-chance exception caught by the debugger, not a real runtime failure, and the Excel file is still written. Use the steps below to confirm which one you are seeing and to stop the debugger from breaking on it.

System.IO.FileNotFoundException: Could not load file or assembly 'IronXL.XmlSerializers'

The message comes from .NET XML serialization probing for an optional serializer assembly that does not exist. The runtime handles the miss internally and continues, so the workbook still saves. With Just My Code disabled, the debugger breaks the moment that internal exception is raised, making a handled event look like a crash.

Solution

1. Reproduce with a minimal workflow

Create a new ASP.NET Core Web Application targeting .NET 10 and install IronXL.Excel version 2026.6.1.

<PackageReference Include="IronXL.Excel" Version="2026.6.1" />
<PackageReference Include="IronXL.Excel" Version="2026.6.1" />
XML

Write a single value and save it to confirm the behavior.

using IronXL;

WorkBook workbook = WorkBook.Create();
WorkSheet sheet = workbook.DefaultWorkSheet;
sheet["A1"].Value = "Hello, World";
workbook.SaveAs("output.xlsx");
using IronXL;

WorkBook workbook = WorkBook.Create();
WorkSheet sheet = workbook.DefaultWorkSheet;
sheet["A1"].Value = "Hello, World";
workbook.SaveAs("output.xlsx");
Imports IronXL

Dim workbook As WorkBook = WorkBook.Create()
Dim sheet As WorkSheet = workbook.DefaultWorkSheet
sheet("A1").Value = "Hello, World"
workbook.SaveAs("output.xlsx")
$vbLabelText   $csharpLabel

2. Enable Just My Code

Open the Visual Studio debugger options and enable Just My Code. When it is off, the debugger breaks on exceptions the library handles internally; turning it on lets those handled exceptions pass without interrupting your session.

3. Confirm the file was generated

Check whether output.xlsx was written to your output path. A successful file tells you the break was a first-chance exception, not a runtime failure.

Please noteA first-chance FileNotFoundException tied to .NET XML serialization can appear even when file generation succeeds.

Debug Tips

There is no standalone IronXL.XmlSerializers NuGet package. Do not try to add one to clear the message; it will not resolve anything and the assembly is not meant to be installed separately.

Curtis Chau
Curtis Chau
  Chat with engineering team now
Technical Writer

Curtis Chau holds a Bachelor’s degree in Computer Science (Carleton University) and specializes in front-end development with expertise in Node.js, TypeScript, JavaScript, and React. Passionate about crafting intuitive and aesthetically pleasing user interfaces, Curtis enjoys working with modern frameworks and creating well-structured, visually appealing manuals.

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