cannot mock a method to return null using LINQ syntax

[mayerc-MSFT]

I need the result of a method vary by argument and return null for a particular argument. I tried the following snippet but Mock.Of fails (exception details further below.)

var mockUserProvider = Mock.Of<IUserProvider>(p => p.GetUserByEmail("john@contoso.com") == new User() && p.GetUserByEmail("alice@fabrikam.com") == null);

If I remove the second clause of the predicate, then the exception doesn't happen.

This feels like something that should work. Could someone offer a work-around?

Not sure if this issue is the same as this: https://groups.google.com/forum/#!topic/moqdisc/tRYXrkgTjI0

exception details

{"Expression of type 'System.Object' cannot be used for parameter of type 'Kobe.Common.Models.Interfaces.IUserWithDetails' of method 'Moq.Language.Flow.IReturnsResult`1[Kobe.Common.Users.IUserProvider] Returns(Kobe.Common.Models.Interfaces.IUserWithDetails)'"}

" at System.Linq.Expressions.Expression.ValidateOneArgument(MethodBase method, ExpressionType nodeKind, Expression arg, ParameterInfo pi)\r\n at System.Linq.Expressions.Expression.ValidateArgumentTypes(MethodBase method, ExpressionType nodeKind, ReadOnlyCollection1& arguments)\r\n at System.Linq.Expressions.Expression.Call(Expression instance, MethodInfo method, IEnumerable1 arguments)\r\n at System.Linq.Expressions.Expression.Call(Expression instance, MethodInfo method, Expression[] arguments)\r\n at Moq.Linq.MockSetupsBuilder.ConvertToSetup(Expression targetObject, Expression left, Expression right)\r\n at Moq.Linq.MockSetupsBuilder.ConvertToSetup(Expression left, Expression right)\r\n at Moq.Linq.MockSetupsBuilder.VisitBinary(BinaryExpression node)\r\n at System.Linq.Expressions.BinaryExpression.Accept(ExpressionVisitor visitor)\r\n at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)\r\n at System.Linq.Expressions.ExpressionVisitor.VisitBinary(BinaryExpression node)\r\n at Moq.Linq.MockSetupsBuilder.VisitBinary(BinaryExpression node)\r\n at System.Linq.Expressions.BinaryExpression.Accept(ExpressionVisitor visitor)\r\n at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)\r\n at System.Linq.Expressions.ExpressionVisitor.VisitLambda[T](Expression1 node)\r\n at System.Linq.Expressions.Expression1.Accept(ExpressionVisitor visitor)\r\n at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)\r\n at System.Linq.Expressions.ExpressionVisitor.VisitUnary(UnaryExpression node)\r\n at Moq.Linq.MockSetupsBuilder.VisitUnary(UnaryExpression node)\r\n at System.Linq.Expressions.UnaryExpression.Accept(ExpressionVisitor visitor)\r\n at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)\r\n at System.Linq.Expressions.ExpressionVisitor.VisitArguments(IArgumentProvider nodes)\r\n at System.Linq.Expressions.ExpressionVisitor.VisitMethodCall(MethodCallExpression node)\r\n at Moq.Linq.MockSetupsBuilder.VisitMethodCall(MethodCallExpression node)\r\n at System.Linq.Expressions.MethodCallExpression.Accept(ExpressionVisitor visitor)\r\n at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)\r\n at Moq.Linq.MockQueryable1.Execute[TResult](Expression expression)\r\n at System.Linq.Queryable.First[TSource](IQueryable1 source, Expression1 predicate)\r\n at Moq.Mock.Of[T](Expression1 predicate)\r\n at Kobe.Web.Controllers.Api.Tests.UserControllerTests.<Patch_Accepts_Addition_Of_Email_Address_Alias_When_User_Has_No_Aliases>d__17.MoveNext() in

[kzu]

Since null is the default return value for such a mock, why even try setting it explicitly?

On Tue, Mar 28, 2017, 9:44 PM mayerc ***@***.***> wrote: I need the result of a method vary by argument and return null for a particular argument. I tried the following snippet but Mock.Of fails (exception details further below.) var mockUserProvider = Mock.Of<IUserProvider>(p => p.GetUserByEmail(" ***@***.***") == new User() && ***@***.***") == null); If I remove the second clause of the predicate, then the exception doesn't happen. This feels like something that should work. Could someone offer a work-around? Not sure if this issue is the same as this: https://groups.google.com/forum/#!topic/moqdisc/tRYXrkgTjI0 exception details {"Expression of type 'System.Object' cannot be used for parameter of type 'Kobe.Common.Models.Interfaces.IUserWithDetails' of method 'Moq.Language.Flow.IReturnsResult`1[Kobe.Common.Users.IUserProvider] Returns(Kobe.Common.Models.Interfaces.IUserWithDetails)'"} " at System.Linq.Expressions.Expression.ValidateOneArgument(MethodBase method, ExpressionType nodeKind, Expression arg, ParameterInfo pi)\r\n at System.Linq.Expressions.Expression.ValidateArgumentTypes(MethodBase method, ExpressionType nodeKind, ReadOnlyCollection1& arguments)\r\n at System.Linq.Expressions.Expression.Call(Expression instance, MethodInfo method, IEnumerable1 arguments)\r\n at System.Linq.Expressions.Expression.Call(Expression instance, MethodInfo method, Expression[] arguments)\r\n at Moq.Linq.MockSetupsBuilder.ConvertToSetup(Expression targetObject, Expression left, Expression right)\r\n at Moq.Linq.MockSetupsBuilder.ConvertToSetup(Expression left, Expression right)\r\n at Moq.Linq.MockSetupsBuilder.VisitBinary(BinaryExpression node)\r\n at System.Linq.Expressions.BinaryExpression.Accept(ExpressionVisitor visitor)\r\n at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)\r\n at System.Linq.Expressions.ExpressionVisitor.VisitBinary(BinaryExpression node)\r\n at Moq.Linq.MockSetupsBuilder.VisitBinary(BinaryExpression node)\r\n at System.Linq.Expressions.BinaryExpression.Accept(ExpressionVisitor visitor)\r\n at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)\r\n at System.Linq.Expressions.ExpressionVisitor.VisitLambda[T](Expression1 node)\r\n at System.Linq.Expressions.Expression1.Accept(ExpressionVisitor visitor)\r\n at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)\r\n at System.Linq.Expressions.ExpressionVisitor.VisitUnary(UnaryExpression node)\r\n at Moq.Linq.MockSetupsBuilder.VisitUnary(UnaryExpression node)\r\n at System.Linq.Expressions.UnaryExpression.Accept(ExpressionVisitor visitor)\r\n at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)\r\n at System.Linq.Expressions.ExpressionVisitor.VisitArguments(IArgumentProvider nodes)\r\n at System.Linq.Expressions.ExpressionVisitor.VisitMethodCall(MethodCallExpression node)\r\n at Moq.Linq.MockSetupsBuilder.VisitMethodCall(MethodCallExpression node)\r\n at System.Linq.Expressions.MethodCallExpression.Accept(ExpressionVisitor visitor)\r\n at System.Linq.Expressions.ExpressionVisitor.Visit(Expression node)\r\n at Moq.Linq.MockQueryable1.Execute[TResult](Expression expression)\r\n at System.Linq.Queryable.First[TSource](IQueryable1 source, Expression1 predicate)\r\n at Moq.Mock.Of[T](Expression1 predicate)\r\n at Kobe.Web.Controllers.Api.Tests.UserControllerTests.<Patch_Accepts_Addition_Of_Email_Address_Alias_When_User_Has_No_Aliases>d__17.MoveNext() in — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#337>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAKW60ZqmMhCdWTX0Vhj8BewiW_sLvi9ks5rqaljgaJpZM4MsXcR> .

--

…

-- /kzu from mobile

[mayerc-MSFT]

hey kzu, thanks for the quick response.

Since null is the default return value for such a mock, why even try
setting it explicitly?

Because it makes the intent clear in the code, therefore the code is easier to read. It's easier to read for myself but more importantly to members of my team, some of which may not have much experience with Moq.

The actual code I'd like to write is more like this:
const User NonExistingUser = null;

var mockUserProvider = Mock.Of<IUserProvider>(p => p.GetUserByEmail("john@contoso.com") == new User() && p.GetUserByEmail("alice@fabrikam.com") == NonExistingUser);

In the above code, the intent (return "non existing user" when GetUserByEmail is invoked with "alice@fabrikam.com") is very clear, hence easier to understand than the assumption that the default is null.

[stakx]

@mayerc-MSFT, you can actually do that!

public partial class User { }

public interface IUserProvider
{
    User GetUserByEmail(string email);
}

…
User nonExistingUser = null;

var provider = Mock.Of<IUserProvider>(p => p.GetUserByEmail("john@contoso.com")   == new User() 
                                        && p.GetUserByEmail("alice@fabrikam.com") == nonExistingUser);

Assert.IsType<User>(provider.GetUserByEmail("john@contoso.com"));
Assert.Equal(nonExistingUser, provider.GetUserByEmail("alice@fabrikam.com"));

(Don't declare nonExistingUser as const, though.)

[stakx]

(Don't declare nonExistingUser as const, though.)

This warning is good for now, but you can ignore it with the next release (version >4.7.63) of Moq.