String operator always returning nonNull string

docs/release-notes/.FSharp.Core/9.0.200.md

@@ -1,9 +1,12 @@
-### Fixed
-
-* Fix exception on Post after MailboxProcessor was disposed ([Issue #17849](https://github.com/dotnet/fsharp/issues/17849), [PR #17922](https://github.com/dotnet/fsharp/pull/17922))
-
-### Added
-
-### Changed
-
-### Breaking Changes
+### Fixed
+
+* Fix exception on Post after MailboxProcessor was disposed ([Issue #17849](https://github.com/dotnet/fsharp/issues/17849), [PR #17922](https://github.com/dotnet/fsharp/pull/17922))
+
+### Added
+
+### Changed
+* String function changed to guarantee a non-null string return type ([PR #17809](https://github.com/dotnet/fsharp/pull/17809))
+
+
+### Breaking Changes
+

src/FSharp.Core/prim-types.fs

@@ -890,11 +890,16 @@ namespace Microsoft.FSharp.Core
                           for m = 0 to len4 - 1 do
                             SetArray4D dst (src1+i) (src2+j) (src3+k) (src4+m) (GetArray4D src i j k m)
 
+        let inline defaultIfNull (defaultStr:string) x =
+            match x with
+            | null -> defaultStr
+            | nonNullString -> nonNullString
+
         let inline anyToString nullStr x = 
             match box x with 
-            | :? IFormattable as f -> f.ToString(null, CultureInfo.InvariantCulture)
+            | :? IFormattable as f -> defaultIfNull nullStr (f.ToString(null, CultureInfo.InvariantCulture))
             | null -> nullStr
-            | _ ->  x.ToString()
+            | _ ->  defaultIfNull nullStr (x.ToString())
 
         let anyToStringShowingNull x = anyToString "null" x
 
@@ -5200,8 +5205,8 @@ namespace Microsoft.FSharp.Core
              when 'T : Guid           = let x = (# "" value : Guid #) in x.ToString(null, CultureInfo.InvariantCulture)
              when 'T struct = 
                 match box value with
-                | :? IFormattable as f -> f.ToString(null, CultureInfo.InvariantCulture)
-                | _ -> value.ToString()
+                | :? IFormattable as f -> defaultIfNull "" (f.ToString(null, CultureInfo.InvariantCulture))
+                | _ -> defaultIfNull "" (value.ToString())
 
              // other common mscorlib reference types
              when 'T : StringBuilder =
@@ -5210,7 +5215,7 @@ namespace Microsoft.FSharp.Core
 
              when 'T : IFormattable =
                 if value = unsafeDefault<'T> then ""
-                else let x = (# "" value : IFormattable #) in x.ToString(null, CultureInfo.InvariantCulture)
+                else let x = (# "" value : IFormattable #) in defaultIfNull "" (x.ToString(null, CultureInfo.InvariantCulture))
 
         [<NoDynamicInvocation(isLegacy=true)>]
         [<CompiledName("ToChar")>]

tests/FSharp.Core.UnitTests/FSharp.Core/OperatorsModule2.fs

@@ -872,6 +872,15 @@ type OperatorsModule2() =
         let result = Operators.string 123.456M
         Assert.AreEqual("123.456", result)
 
+        let result = Operators.string { new obj () with override _.ToString () = null }
+        Assert.AreEqual("", result)
+
+        let result = Operators.string { new obj () with override _.ToString () = Operators.string null }
+        Assert.AreEqual("", result)
+
+        let result = Operators.string { new IFormattable with override _.ToString (_, _) = null }
+        Assert.AreEqual("", result)
+
         // Following tests ensure that InvariantCulture is used if type implements IFormattable
 
         // safe current culture, then switch culture