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In .NET 5 RC2 and later versions, projects no longer define preprocessor symbols for earlier versions, but only for the version that they target. This is the same behavior as .NET Core 1.0 - 3.1.
Version introduced
5.0 RC2
Change description
In .NET 5 preview 7 through RC1, projects that target net5.0 define both NETCOREAPP3_1 and NET5_0 preprocessor symbols. The intent behind this behavior change was that starting with .NET 5, conditional compilation symbols would be cumulative.
In .NET 5 RC2 and later, projects only define symbols for the target framework monikers (TFM) that it targets and not for any earlier versions.
Reason for change
The change from preview 7 was reverted due to customer feedback. Defining symbols for earlier versions surprised and confused customers, and some assumed it was a bug in the C# compiler.
Recommended action
Make sure that your #if logic doesn't assume that NETCOREAPP3_1 is defined when the project targets net5.0 or higher. Instead, assume that NETCOREAPP3_1 is only defined when the project explicitly targets netcoreapp3.1.
For example, if your project multitargets for .NET Core 2.1 and .NET Core 3.1 and you call APIs that were introduced in .NET Core 3.1, your #if logic should look as follows:
#if NETCOREAPP2_1 || NETCOREAPP3_0
// Fallback behavior for old versions.
#elif NETCOREAPP
// Behavior for .NET Core 3.1 and later.
#endif
Affected APIs
N/A
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