Detect DbFunction returning a nullable value expression

src/EFCore.Relational/Properties/RelationalStrings.resx

@@ -217,6 +217,9 @@
  <data name="DbFunctionNonScalarCustomTranslation" xml:space="preserve">
  <value>Cannot set custom translation on the DbFunction '{function}' since it is not a scalar function.</value>
  </data>
+ <data name="DbFunctionNullableValueReturnType" xml:space="preserve">
+ <value>The DbFunction '{function}' returns a SqlExpression of type '{type}', which is a nullable value type. DbFunctions must return expressions with non-nullable value types, even when they may return null.</value>
+ </data>
  <data name="DefaultValueSqlUnspecified" xml:space="preserve">
  <value>The default value SQL has not been specified for the column '{table}.{column}'. Specify the SQL before using Entity Framework to create the database schema.</value>
  </data>

src/EFCore.Relational/Query/RelationalMethodCallTranslatorProvider.cs

@@ -60,8 +60,17 @@ public RelationalMethodCallTranslatorProvider(RelationalMethodCallTranslatorProv
  {
  if (dbFunction.Translation != null)
  {
- return dbFunction.Translation.Invoke(
+ var translation = dbFunction.Translation.Invoke(
  arguments.Select(e => _sqlExpressionFactory.ApplyDefaultTypeMapping(e)).ToList());
+
+ if (translation.Type.IsNullableValueType())
+ {
+ throw new InvalidOperationException(
+ RelationalStrings.DbFunctionNullableValueReturnType(
+ dbFunction.ModelName, dbFunction.ReturnType.ShortDisplayName()));
+ }
+
+ return translation;
  }
 
  var argumentsPropagateNullability = dbFunction.Parameters.Select(p => p.PropagatesNullability);

test/EFCore.Relational.Specification.Tests/Query/UdfDbFunctionTestBase.cs

@@ -224,6 +224,9 @@ public static string IdentityStringNonNullable(string s)
  public static string IdentityStringNonNullableFluent(string s)
  => throw new Exception();
 
+ public static int? NullableValueReturnType()
+ => throw new NotImplementedException();
+
  public string StringLength(string s)
  => throw new Exception();
 
@@ -310,6 +313,14 @@ protected override void OnModelCreating(ModelBuilder modelBuilder)
  negated: false,
  typeMapping: null));
 
+ modelBuilder.HasDbFunction(typeof(UDFSqlContext).GetMethod(nameof(NullableValueReturnType), Array.Empty<Type>()))
+ .HasTranslation(
+ _ => new SqlFunctionExpression(
+ "foo",
+ nullable: true,
+ typeof(int?),
+ typeMapping: null));
+
  //Instance
  modelBuilder.HasDbFunction(typeof(UDFSqlContext).GetMethod(nameof(CustomerOrderCountInstance)))
  .HasName("CustomerOrderCount");
@@ -1009,6 +1020,23 @@ public virtual void Scalar_Function_with_nested_InExpression_translation()
  Assert.Equal(4, query.Count);
  }
 
+#if RELEASE
+ [ConditionalFact]
+ public virtual void Scalar_Function_with_nullable_value_return_type_throws()
+ {
+ using var context = CreateContext();
+
+ var exception = Assert.Throws<InvalidOperationException>(
+ () => context.Customers.Where(c => c.Id == UDFSqlContext.NullableValueReturnType()).ToList());
+
+ Assert.Equal(
+ RelationalStrings.DbFunctionNullableValueReturnType(
+ "Microsoft.EntityFrameworkCore.Query.UdfDbFunctionTestBase<Microsoft.EntityFrameworkCore.Query.UdfDbFunctionSqlServerTests+SqlServer>+UDFSqlContext.NullableValueReturnType()",
+ "int?"),
+ exception.Message);
+ }
+#endif
+
  #endregion
 
  #region Instance