# Problems with referencing C# defined custom operator from F# **URL:** \ **Category:** General\ **Created:** [November 7, 2023, 1:07pm UTC](https://forums.fsharp.org/t/problems-with-referencing-c-defined-custom-operator-from-f/3401 "2023-11-07T13:07:39Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![htc](https://avatars.discourse-cdn.com/v4/letter/h/a9adbd/32.png) [@htc](https://forums.fsharp.org/u/htc)\ **Post date:** [November 7, 2023, 1:07pm UTC](https://forums.fsharp.org/t/problems-with-referencing-c-defined-custom-operator-from-f/3401/1 "2023-11-07T13:07:39Z")
Dear community, I have a problem and my preferred solution does not work: 1. Problem: There is a C# library with an overloaded == operator [gurobi 9.1](https://www.gurobi.com/documentation/9.1/refman/cs_oper_equal.html). I want to use this operator in F#, such as in ```auto let setEqual (lhs : GRBLinExpr, rhs : GRBLinExpr) = GRBLinExpr.(==) (lhs, rhs) ``` This fails with “FS0039 The type ‘GRBLinExpr’ does not define the field, constructor or member ‘==’.” Also addressing it as member op\_EqualsEquals as outlined in [F# ops](https://learn.microsoft.com/en-us/dotnet/fsharp/language-reference/operator-overloading) does lead to the same error. 1. Extra information: Adding the library to a C# project shows me: ```auto public static Gurobi.GRBTempConstr operator ==(Gurobi.GRBLinExpr lhs, Gurobi.GRBLinExpr rhs) Member of Gurobi.GRBLinExpr ) ``` and the following compiles ```auto static GRBTempConstr SetEqual(GRBLinExpr lhs, GRBLinExpr rhs) { return lhs == rhs; } ``` 1. Possible workaround: I can create an extra C# project, wrap the == operator there in a static method and reuse this method in F#. Pro: Works. Cons: extra project. So my questions are: a) Why does F# not allow me to use this operator? Are there some limitations to using operators from F#? The F# docs does not mention any (sorry, see “F# ops” link above, as this forum does not let me post the same link twice :-() . b) Are there other workarounds than 3)? Maybe with reflection? Cordially, htc ---
**Author:** ![ntwilson](https://yyz2.discourse-cdn.com/flex030/user_avatar/forums.fsharp.org/ntwilson/32/984_2.png) [@ntwilson](https://forums.fsharp.org/u/ntwilson)\ **Post date:** [November 7, 2023, 4:44pm UTC](https://forums.fsharp.org/t/problems-with-referencing-c-defined-custom-operator-from-f/3401/2 "2023-11-07T16:44:44Z")
I think you can do it with `GRBLinExpr.op_Equality`. The table in that F# ops link of yours lists that for the `=` operator and not `==`, but for C# code, I think it uses `==` everywhere that F# uses `=`. At least in my testing, I can have a C# class: ```auto public class Class1 { private int A; public Class1(int a) => A = a; public static bool operator ==(Class1 a, Class1 b) => a.A == b.A; public static bool operator !=(Class1 a, Class1 b) => a.A != b.A; } ``` and then from F# I can do ```auto printfn "%A" (Class1.op_Equality (Class1 6, Class1 6)) ``` ---
**Author:** ![abelbraaksma](https://yyz2.discourse-cdn.com/flex030/user_avatar/forums.fsharp.org/abelbraaksma/32/1114_2.png) [@abelbraaksma](https://forums.fsharp.org/u/abelbraaksma)\ **Post date:** [November 15, 2023, 12:53pm UTC](https://forums.fsharp.org/t/problems-with-referencing-c-defined-custom-operator-from-f/3401/3 "2023-11-15T12:53:13Z")
I just downloaded that library you mentioned and checked the documentation from the URL you gave and keep in mind that the equality operator here **does not return a boolean**. In fact, it is a `GRBTempConstr`. In other words, if you try to use this in a context where equality is expected, it won’t work. > a) Why does F# not allow me to use this operator? As @ntwilson already showed, it is not the `op_EqualsEquals`, but the `op_Equality` that you are after. C# is confusing here: when C# uses `==`, it calls the `Equals` overload, or the `op_Equality`, if it is defined, not the `op_EqualsEquals`… > Are there some limitations to using operators from F#? The F# docs does not mention any No. So, even though the result here is not a boolean, you can use it. * * * It is important to realize that when you use the `=` (equality) operator in F#, it will do structural comparison. In other words, even if you have the `op_Equality` defined, this will not be called automatically. If we slightly adjust your code, it compiles: ```auto let setEqual (lhs: GRBLinExpr, rhs: GRBLinExpr) = GRBLinExpr.(=) (lhs, rhs) ``` Or: ```auto let setEqual (lhs: GRBLinExpr, rhs: GRBLinExpr) = GRBLinExpr.op_Equality (lhs, rhs) ``` As you can see, you needed `op_Equality` here (which is what `operator (==)` compiles to from C#) and not `op_Equals` (which, btw, is not actually listed in that F# Operator list that you linked). If we look at the compiled MSIL code in ILSpy, we see the same. The actual `operator (==)` from C# is compiled into a static member `op_Equality` and not `op_Equals`. However, you are right in that, to map the F# operator `==` (double-equals), you need to define an operator `op_EqualsEquals`, which you can then call the same way you do for C#, if you would like that (that is, as `lhs == rhs`). ---
**Author:** ![htc](https://avatars.discourse-cdn.com/v4/letter/h/a9adbd/32.png) [@htc](https://forums.fsharp.org/u/htc)\ **Post date:** [December 4, 2023, 7:25am UTC](https://forums.fsharp.org/t/problems-with-referencing-c-defined-custom-operator-from-f/3401/4 "2023-12-04T07:25:53Z")
Thanks @ntwilson! I so rarely write C# code, and fixated on translating == into F# as ==, not =. Of course I found nothing. Thanks for unblocking me! ---
**Author:** ![htc](https://avatars.discourse-cdn.com/v4/letter/h/a9adbd/32.png) [@htc](https://forums.fsharp.org/u/htc)\ **Post date:** [December 4, 2023, 7:27am UTC](https://forums.fsharp.org/t/problems-with-referencing-c-defined-custom-operator-from-f/3401/5 "2023-12-04T07:27:22Z")
@abelbraaksma. Thank you for the pointer to using custom = when using F# native types (where structural comparison is automatically implemented by default). That is not the case here, but nice to remember for the next internal DSL I write.