The new TimeSpan FromMilliseconds overload breaks System.Linq.Expressions

[Tragetaschen]

edited by @tarekgh to add the API proposal.

Description

In .NET 9.0, we introduced overloads for the TimeSpan.From... methods to accept integral types. Unfortunately, the approved proposal included only a single overload with an optional parameter for the FromMilliseconds method.

public static TimeSpan FromMilliseconds(long milliseconds, long microseconds = 0);

Unfortunately, this broke the LINQ expression when used as follows, resulting in the error: error CS0854: An expression tree may not contain a call or invocation that uses optional arguments.

Expression<Action> a = () => TimeSpan.FromMilliseconds(1000); // compiles with .NET 8, fails with .NET 9

Note, this worked fine in net8 because we didn't have the new overload and we had the one that is accepting double parameter.

Proposal

public partial struct TimeSpan
{
+    public static TimeSpan FromMilliseconds(long milliseconds);

+    public static TimeSpan FromMilliseconds(long milliseconds, long microseconds);
-    public static TimeSpan FromMilliseconds(long milliseconds, long microseconds = 0);
}

Original Post

Description

I have seen the breaking change notice with regards to F#, but migrating to .NET 9 has revealed a couple of places in our C# projects, where constructing a TimeSpan is part of a System.Linq.Expressions.Expression. These now don't compile with .NET 9

Reproduction Steps

using System.Linq.Expressions;
Expression<Action> a = () => TimeSpan.FromMilliseconds(1000); // compiles with .NET 8, fails with .NET 9

Expected behavior

Code continues to compile

Actual behavior

Code breaks in .NET 9

Regression?

yes

Known Workarounds

You need to force the double overload

Expression<Action> a = () => TimeSpan.FromMilliseconds(1000.0);

or provide the optional parameter

Expression<Action> a = () => TimeSpan.FromMilliseconds(1000, 0); 

Configuration

No response

Other information

The reason is that the optional parameters are not supported in expressions.

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[Tragetaschen]

Introduced with #93890

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[julealgon]

I was under the impression that exposing methods with default parameters in public APIs was frowned upon, and that it was best to just have multiple overloads instead.

Does anybody know if that's not really the case, or if it still applies, why these new APIs for TimeSpan didn't follow that practice?

[Tragetaschen]

The approved API said

The methods should be overloaded to have no default parameters per our normal default parameters usability guideline

But the actual API missed the milliseconds overload and consequently didn't implement it in #98633.

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