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SYSLIB0026: X509Certificate and X509Certificate2 are immutable

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The following mutable x509 certificate APIs are marked as obsolete, starting in .NET 6. Using these APIs in code generates warning SYSLIB0026 at compile time.

Workarounds

Create a new instance of X509Certificate and X509Certificate2 using a constructor overload that accepts the certificate as input. For example:

// Change this:
cert.Import("/path/to/certificate.crt");

// To this:
cert.Dispose();
cert = new X509Certificate2("/path/to/certificate.crt");

Suppress a warning

If you must use the obsolete APIs, you can suppress the warning in code or in your project file.

To suppress only a single violation, add preprocessor directives to your source file to disable and then re-enable the warning.

// Disable the warning.
#pragma warning disable SYSLIB0026

// Code that uses obsolete API.
// ...

// Re-enable the warning.
#pragma warning restore SYSLIB0026

To suppress all the SYSLIB0026 warnings in your project, add a <NoWarn> property to your project file.

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
   ...
   <NoWarn>$(NoWarn);SYSLIB0026</NoWarn>
  </PropertyGroup>
</Project>

For more information, see Suppress warnings.