Coalesce Error: Not to be used directly - use inside QueryOver expression

[fairking]

Could someone please look if it's a bug?
NHibernate 5.2.6 (net.core)
Dialect: NHibernate.Dialect.MsSql2008Dialect
Driver: NHibernate.Driver.SqlClientDriver,NHibernate
Db: MSSQL 14.0.2027.2
The following query throws an exception:

queryOver
    .Select(new[]
    {
        Projections.Property(() => root.Id).WithAlias(() => vm.Id),
        Projections.Property(() => root.FullName).Coalesce(Projections.Property(() => root.UserName)).WithAlias(() => vm.Name),
    });

The exception is:

System.InvalidOperationException: Not to be used directly - use inside QueryOver expression
   at NHibernate.Criterion.ProjectionsExtensions.Coalesce[T](T objectProperty, T replaceValueIfIsNull)

[bahusoid]

No. Error clearly says that Coalesce is expected to be used only inside expression (lambda C# expression). Like:
.Select( () => root.FullName.Coalesce(root.UserName))

It won't work with projections.

Though it seems you can use SelectList for your needs . Something like:

queryOver
    .SelectList(l => 
        l.Select(() => root.Id).WithAlias(() => vm.Id)
         .Select(() => root.FullName.Coalesce(root.UserName)).WithAlias(() => vm.Name));

[bahusoid]

Hm... Actually my suggestion won't work in 5.2 without #2246 and #2157.

So as a workaround you can adjust your existing code to

new SqlFunctionProjection("coalesce", returnType, projection1, projection2);

Or simply select both properties and do coalescing logic in code.