Class ComValue

ComValue is a wwDotnetBridge .NET helper class that wraps problem .NET types that cannot be accessed in FoxPro due to COM Type Limitations. COM doesn't support many .NET types and this class allows you to use them with this wrapper object that can be passed and returned to and from the explicit invocation methods like InvokeMethod(), SetProperty(), GetProperty(), which know how to fix up the ComValue structure into real .NET types and vice versa.

This applies both to inbound processing as a method parameter or property set value, as well as on return values. For example, the Long type cannot be passed over COM. If a method requires a Long parameter, you can create a ComValue instance and use .SetLong(lnValue) to pass the value to InvokeMethod(). When the call returns and it also returns a Long, InvokeMethod() automatically returns a ComValue object from which you can retrieve the value .GetLong() (or .GetInt64()).

Important: ComValue fixup works only with the wwDotnetBridge intrinsic methods. It's important to understand that it's wwDotnetBridge that understands ComValue instances, not .NET so you can only pass or receive a ComValue through the indirect access methods, never by accessing the .NET COM instance directly.

Examples of unsupported types include:

  • Long, Single, Decimal number types
  • Guid, Byte, DbNull, char
  • Any Value type (based on struct in C#)
  • Enum Values
  • Access to any Generic Value or Type

That's a pretty wide swath of types that are inaccessible via direct instance COM access, but with the help of the ComValue class it's still possible to access these types in FoxPro.

How it works

ComValue works by creating a .NET wrapper object with a Value property that holds the actual .NET value and methods that allow setting and retrieving that value - or a translation thereof - in FoxPro. The Value is stored in .NET and is never passed directly to FoxPro. Instead you pass or receive a ComValue instance that contains the Value and has conversion routines that allow convert the value between .NET and FoxPro types.

Automatic ComValues

ComValue results are automatically returned for known problem types with:

  • GetProperty()
  • InvokeMethod() result values

For example, if you call a method that returns a Guid value, GetProperty() or InvokeMethod() return a ComValue object and you can call GetGuid() to retrieve the Guid as a string.

loGuid = loBridge.InvokeMethod(loObj,"GetGuid") 
* Get Guid from ComValue
lcGuid = loGuid.GetGuid()

You can pass ComValue objects when using these methods:

  • SetProperty()
  • InvokeMethod() parameters
  • CreateInstance() constructor parameters
  • ComArray.AddItem()

For these methods you create a ComValue instance and set the value and then pass that to one of the above methods.

lcGuid = GetAGuidStringFromSomewhere()
loGuid = loBridge.CreateValue()

llResult = loBridge.InvokeMethod(loObj,"SetGuid",loGuid)

Simple type conversion:

*** Create .NET Object instance
loNet = loBridge.CreateInstance("MyApp.MyNetObject")

*** Convert the 'unsupported' parameter type
LOCAL loVal as Westwind.WebConnection.ComValue
loVal = loBridge.CreateComValue()

*** Call method that takes Int16 parameter

ComValue caching for Method and Property Invocation

This class also supports setting a ComValue from properties and method results. This is useful if you have a method or property that uses a type inaccessible via COM (like strongly typed or subclassed dataset objects for example). In this case you can call the SetValueXXX methods to fill the ComValue structure and then use this ComValue in InvokeMethod, SetProperty calls which automatically pick up this ComValue object's underlying .NET type.

*** Create an array of parameters (ComArray instance)
loParms = loBridge.CreateArray("System.Object")
loParms.AddItem("Error Message")

*** Create a ComValue structure to hold the result: a DataSet
LOCAL loValue as Westwind.WebConnection.ComValue
loValue = loBridge.CreateComValue()

*** Invoke the method and store the result on the ComValue structure
*** Result from this method is DataSet which can't be marshalled properly over COM
? loValue.SetValueFromInvokeMethod(loService,"Login",loParms)

*** This is your raw DataSet
*? loValue.Value   && direct access won't work  because it won't marshal

*** Now call a method that requires the DataSet parameter

The jist of this is that the DataSet result is never passed through FoxPro code, but is stored in ComValue and then that ComValue is used as a parameter in the InvokeMethod call. All indirect execution methods (InvokeMethod,SetProperty etc.) understand ComValue and use the Value property for the parameter provided.

For more detailed information check out this blog post:

public class ComValue : object

Class Members




Returns a Guid stored in the Value property. This is necessary because Guids are Value types that cannot be passed into or be accessed by FoxPro code at all.



Returns the name of the actual .NET type (not what FoxPro converts it to) actually stored on the .Value property.



Retrieves the fixed up value from the .Value property. The call is returned by calling .GetProperty() to retrieve the value which automatically converts problem types to the appropriate wrapper types (ie. Array/Collections to ComArrays, DbNulls to Nulls etc.).



Creates a new GUID and stores it in the Value and also returns the new Guid as a string.



Returns a byte value which is similar to Int16.

public void SetByte(object value);


Converts a FoxPro string or number to a .NET char value on the Value structure.

o.ComValue.SetChar(object val)


Sets a DbNull value from FoxPro. DbNull is a value type and can't be accessed directly in FoxPro.



Sets a Decimal value. Turns any FoxPro numeric value to a .NET decimal value.

public void SetDecimal(object val);


Allows you to create an Enum value that can be passed as a parameter.

o.ComValue.SetEnum(string enumString)


Sets the value to a .NET Guid by passing in either a GUID string, a ComGuid object, or a null (which creates a new Guid).

o.ComValue.SetGuid(object value)


Sets a Short value which is not supported in Visual FoxPro

public void SetInt16(object val);


Sets a Int64 value from a FoxPro numeric value. Since FoxPro doesn't have a cast to 64 bit values natively you can use decimal or float values in Fox or 32 bit integers.

public void SetInt64(object val);


Turns a FoxPro number to a Long 64 bit integer value in .NET. Note that FoxPro doesn't support casting to a 64 bit value, so you can pass a 32 bit integer or numbers that are decimal or floats to this function to achieve 64 bit Long values.

public void SetLong(object val);


Sets the value of this object to a Single based on a FoxPro number passed in.



Sets the value of this object to a .NET Single/float based on a FoxPro number passed in.



Sets a 32 bit UInt value on the Value property.

o.ComValue.SetUInt34(object val)


Sets a 64 bit unsigned integer value from a FoxPro number.

o.ComValue.SetUInt64(object val)


This method creates an instance of a generic type on the ComValue structure by allowing you to specify a generic type name, the generic type parameters (as a ComArray) and optional constructor parameters (as a ComArray.)



Allows setting of the Value property from the result of an object instantiation.

o.ComValue.SetValueFromCreateInstance(string typeName,ComArray parms)


Sets the ComValue structure to the specified Enum value.

o.ComValue.SetValueFromEnum(string enumType, string enumKeyName)


Sets the Value property from a method call that passes it's positional arguments as an array.

public void SetValueFromInvokeMethod(object objectRef,     string method,     ComArray parms);


Sets the Value property from a method call that passes it's positional arguments as an array.

public void SetValueFromInvokeMethod(string typename,     string method,     ComArray parms);


Sets the Value property from a property retrieved from .NET Useful to transfer value in .NET that are marshalled incorrectly in FoxPro such as Enum values (that are marshalled as numbers).

public void SetValueFromProperty(object objectRef,     string property);


Sets the value property from a static property retrieved from .NET. Useful to transfer value in .NET that are marshalled incorrectly in FoxPro such as Enum values (that are marshalled as numbers)

public void SetValueFromStaticProperty(string typeName,     string property);


Allows you to call a method on the System.Convert (as a string method name) and pass a single value that is then converted and stored on the Value member.

o.ComValue.SetValueFromSystemConvert(string method, object value)


Returns the value of contained ComValue instance value as a string.



Internally this value is set by the various SetXXX methods. It's of type objcect but set to the appropriate .NET subtype.


Namespace: Westwind.WebConnection
Assembly: wwdotnetbridge.dll

© West Wind Technologies, 1996-2024 • Updated: 10/21/2022
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