Passing Values by Reference

On occasion you'll find .NET APIs that pass parameters by reference. Thankfully by ref parameters are rare since it's much easier to return objects then return values via parameters, but nevertheless there are a few APIs that require ref or out parameters.

Making these calls is not quite obvious with wwDotnetBridge. There are essentially two ways that you can make these calls:

  • Using direct COM Access of .NET objects
  • Using ComValue Parameters in InvokeMethod/InvokeStaticMethod calls

Direct Call Syntax

You can pass and retrieve reference parameters in .NET easily if you're using direct call syntax. Assume the following function signature:

public void PassByReference(ref int intValue, string stringValue, ref decimal decimalValue)
{
    intValue = intValue*2;
    decimalValue = decimalValue*2;
    stringValue += " Updated!";
}

And you can easily call it with code like the following:

loNet = loBridge.Createinstance("Westwind.WebConnection.TypePassingTests")
lnInt = 10
lnDecimal = 5.22
lcString = "Hello World."
loNet.PassByReference(@lnInt,lcString,@lnDecimal)
? lnInt,lnDecimal,lcString

Note that I'm using direct access of the function I am calling (loNet.PassByReference() call) and COM handles mapping the ref parameters automatically.

Passing Arrays by Reference

To pass arrays by reference works the same way as above although it's not quite as obvious. You still need to assign the array to a ComValue structure, but most importantly you need to apply the array explictly to the value structure:

loStrings = loBridge.CreateArray("System.String")
loStrings.AddItem("It's")
loStrings.AddItem("BigDay")

LOCAL loValue as Westwind.WebConnection.ComValue
loValue = loBridge.CreateComValue()
loBridge.SetProperty(loValue,"Value",loStrings)

lobridge.InvokeMethod(loNet,"PassArrayByReference",loValue)

loStringsResult =  loBridge.GetProperty(loValue,"Value")
? loStringsResult.Count

Using ComValue with InvokeMethod/InvokeMethodStatic

First lets show what does not work:

loNet = loBridge.CreateInstance("Westwind.WebConnection.TypePassingTests")
lnInt = 10
lnDecimal = 5.22
lcString = "Hello World."
loNet.InvokeMethod(loNet,"PassByReference",@lnInt,lcString,@lnDecimal)
? lnInt,lnDecimal,lcString

The reason this doesn't work is that indirect calls pass the parameters through 2 additional API layers and in that process the ByRef linkage is lost.

To get around this you can use the ComValue class to hold your ByRef parameters. ComValue parameters pass their value to a function and also receive that value back on function call completion effectively providing the semblance of ByRef parameters. The syntax to do this is a little more verbose.

Assume you have a method like this:

public void PassByReference(ref int intValue, string stringValue, ref decimal decimalValue)
{
    intValue = intValue*2;
    decimalValue = decimalValue*2;
    stringValue += " Updated!"; 
}

Note that 2 of the parameters that are numeric are passed by reference.

To call this method from FoxPro looks like this:

LOCAL loNet as Westwind.WebConnection.TypePassingTests
loNet = loBridge.Createinstance("Westwind.WebConnection.TypePassingTests")

*** Pass parameters by Reference

*** Create ComValue objects for each parameter
loInt = loBridge.CreateComValue()
loInt.Value = INT(10)

loDecimal = loBridge.CreateComValue()
loDecimal.Value = CAST( 5.22 as Currency)

? "Original:"
? loInt.Value, loDecimal.Value

*** Call the method and pass ComArray parameter
loBridge.InvokeMethod(loNet,;
                            "PassByReference",;
                            loInt,"Hello World",loDecimal)

*** Look at the result values
? "Updated:"
? loInt.Value, loDecimal.Value

Notice the use of the ComValue class to assign a value and then also retrieving the value back after the call is complete. If ComValue is used for a parameter wwDotnetBridge always updates the ComValue.Value property with the updated value after the call is complete. You can read this value and treat it as if it was passed by directly through a ByRef assignment.


© West Wind Technologies, 1996-2019 • Updated: 05/05/16
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