# Package validation overview

Cross-platform compatibility has become a mainstream requirement for .NET library authors. However, without validation tooling for these packages, they often don't work well. This is especially problematic for emerging platforms where adoption isn't high enough to warrant special attention by library authors.

Until package validation was introduced, the .NET SDK tooling provided almost no validation that multi-targeted packages were well formed. For example, a package that multi-targets for .NET 6 and .NET Standard 2.0 needs to ensure that code compiled against the .NET Standard 2.0 binary can run against the .NET 6 binary.

You might think that a change is safe and compatible if source consuming that change continues to compile without changes. However, the changes can still cause problems at run time if the consumer wasn't recompiled. For example, adding an optional parameter to a method or changing the value of a constant can cause these kinds of compatibility issues.

Package validation tooling allows library developers to validate that their packages are consistent and well formed. It provides the following checks:

• Validates that there are no breaking changes across versions.
• Validates that the package has the same set of publics APIs for all the different runtime-specific implementations.
• Helps developers catch any applicability holes.

## Enable package validation

You enable package validation in your .NET project by setting the EnablePackageValidation property to true.

<Project Sdk="Microsoft.NET.Sdk">

<PropertyGroup>
<TargetFrameworks>netstandard2.0;net6.0</TargetFrameworks>
<EnablePackageValidation>true</EnablePackageValidation>
</PropertyGroup>

</Project>


EnablePackageValidation runs a series of checks after the Pack task. There are some additional checks that can be run by setting other MSBuild properties.

## Validator types

There are three different validators that verify your package as part of the Pack task:

• The Baseline version validator validates your library project against a previously released, stable version of your package.
• The Compatible runtime validator validates that your runtime-specific implementation assemblies are compatible with each other and with the compile-time assemblies.
• The Compatible framework validator validates that code compiled against one framework can run against all the others in a multi-targeting package.

## Suppress compatibility errors

To suppress compatibility errors for intentional changes, add a CompatibilitySuppressions.xml file to your project. You can generate this file automatically by passing /p:GenerateCompatibilitySuppressionFile=true if you're building the project from the command line, or by adding the following property to your project: <GenerateCompatibilitySuppressionFile>true</GenerateCompatibilitySuppressionFile>

The suppression file looks like this.

<?xml version="1.0" encoding="utf-8"?>
<Suppressions xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<Suppression>
<DiagnosticId>CP0002</DiagnosticId>
<Target>M:A.B.DoStringManipulation(System.String)</Target>
<Left>lib/netstandard2.0/A.dll</Left>
<Right>lib/net6.0/A.dll</Right>
<IsBaselineSuppression>false</IsBaselineSuppression>
</Suppression>
</Suppressions>

• DiagnosticId specifies the ID of the error to suppress.

• Target specifies where in the code to suppress the diagnostic IDs

• Left specifies the left operand of an APICompat comparison.

• Right specifies the right operand of an APICompat comparison.

• IsBaselineSuppression: set to true to apply the suppression to a baseline validation; otherwise, set to false.