How can we return error value from ReflectionFunction?

[exceedsystem]

Is there a way to return an error from a custom function that returns such as a StringValue?

Example:

using Microsoft.PowerFx;
using Microsoft.PowerFx.Types;

var config = new PowerFxConfig();

config.AddFunction(new MyFuncFunction());

var engine = new RecalcEngine(config);

var ret = engine.Eval("If(IsError(MyFunc(\"\")), \"Error\", \"Success\")");

public class MyFuncFunction : ReflectionFunction
{
  public StringValue Execute(StringValue param)
  {
    if (param.Value == "")
    {
      // Failed
      // How to return error from here?
    }
    else
    {
      // Success
      return FormulaValue.New("");
    }
  }
}

Best regards.

[anderson-joyle]

Check the CommomErrors class. You can call some of the static methods to get a pre-built error message or you can call CommomErrors.CustomError to build your own custom error message.
If none of them work for you, it is still a good source to learn how to build ErrorValue objects.

As a reminder, it is also possible to create a ErrorValue from an expression:
Error({Kind:ErrorKind.InvalidArgument, Message:"This argument is invalid."})

[exceedsystem]

Thanks for the valuable information, I will check it out.

[exceedsystem]

I searched the Power Fx source with what you told me and was able to return the error in the following way.

Power-Fx/src/tools/Repl/Program.cs

Line 493 in a48e1a7

public FormulaValue Execute(StringValue fileNameSV)

Example:

using System;

using Microsoft.PowerFx;
using Microsoft.PowerFx.Types;

var config = new PowerFxConfig();

config.AddFunction(new MyFunc());

var engine = new RecalcEngine(config);

// Has a error
var ret1 = engine.Eval("If(IsError(MyFunc(\"\")), \"Error\", \"Success\")") as StringValue;
Console.WriteLine($"{ret1.Value}");

// No error
var ret2 = engine.Eval("If(IsError(MyFunc(\"Foo\")), \"Error\", \"Success\")") as StringValue;
Console.WriteLine($"{ret2.Value}");

public class MyFunc : ReflectionFunction
{
    // Declare the return type in the base constructor
    public MyFunc()
        : base("MyFunc", FormulaType.String, FormulaType.String)
    {
    }

    // Make the return value a FormulaValue instead of a StringValue
    public FormulaValue Execute(StringValue param)
    {
        if (param.Value.Length == 0)
        {
            //  In case of an error, return an ErrorValue
            return ErrorValue.NewError(new ExpressionError
            {
                Kind = ErrorKind.InvalidArgument,
                Severity = ErrorSeverity.Critical,
                Message = "Invalid parameter."
            });
        }
        else
        {
            // Return value on success
            return FormulaValue.New("");
        }
    }
}

Result:

Error
Success

Thank you so much.

[anderson-joyle]

Awesome!

[MikeStall]

One nit, errors can be typed. So you should use the 2nd constructor function and pass the type in:

return ErrorValue.NewError(new ExpressionEror {... }, FormulaValue.String)

Also curious - can you tell me more about your scenario?

[exceedsystem]

@MikeStall

One nit, errors can be typed. So you should use the 2nd constructor function and pass the type in:
return ErrorValue.NewError(new ExpressionEror {... }, FormulaValue.String)

Thank you for your advice.

I don't have a specific senario yet, as I am still learning and experimenting how to implement Power Fx features in our products.

[MikeStall]

FYI - here's an improvement here: #1018
See example there- now you can just throw a special exception and it will get converted to properly typed error.