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.