Expressions (WF)

A Windows Workflow Foundation (WF) expression is any activity that returns a result. All expression activities derive indirectly from Activity, which contains an OutArgument property named Result as the activity’s return value. WF ships with a wide range of expression activities from simple ones like VariableValue and VariableReference, which provide access to single workflow variable through operator activities, to complex activities such as VisualBasicReference and VisualBasicValue that offer access to the full breadth of Visual Basic language to produce the result. Additional expression activities can be created by deriving from CodeActivity or NativeActivity.

Using Expressions

Workflow designer uses VisualBasicValue and VisualBasicReference for all expressions in Visual Basic projects, and CSharpValue and CSharpReference for expressions in C# workflow projects.

Workflows produced by designer are saved in XAML, where expressions appear enclosed in square brackets, as in the following example.

  
    
    
    
    
  
  
    
      [r]
    
    
      [a + b + c]
    
  

When defining a workflow in code, any expression activities can be used. The following example shows the usage of a composition of operator activities to add three numbers:

Variable a = new Variable("a", 1);
Variable b = new Variable("b", 2);
Variable c = new Variable("c", 3);
Variable r = new Variable("r", 0);

Sequence w = new Sequence
{
    Variables = { a, b, c, r },
    Activities =
    {
        new Assign {
            To = new OutArgument(r),
            Value = new InArgument {
                Expression = new Add {
                    Left = new Add {
                        Left = new InArgument(a),
                        Right = new InArgument(b)
                    },
                    Right = new InArgument(c)
                }
            }
        }
    }
};

The same workflow can be expressed more compactly by using C# lambda expressions, as shown in the following example:

Variable a = new Variable("a", 1);
Variable b = new Variable("b", 2);
Variable c = new Variable("c", 3);
Variable r = new Variable("r", 0);

Sequence w = new Sequence
{
    Variables = { a, b, c, r },
    Activities =
    {
        new Assign {
            To = new OutArgument(r),
            Value = new InArgument((ctx) => a.Get(ctx) + b.Get(ctx) + c.Get(ctx))
        }
    }
};

Extending Available Expressions with Custom Expression Activities

Expressions in .NET Framework 4.6.1 are extensible allowing for additional expression activities to be created. The following example shows an activity that returns a sum of three integer values.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Activities;

namespace ExpressionsDemo
{
    public sealed class AddThreeValues : CodeActivity
    {
        public InArgument Value1 { get; set; }
        public InArgument Value2 { get; set; }
        public InArgument Value3 { get; set; }

        protected override int Execute(CodeActivityContext context)
        {
            return Value1.Get(context) +
                   Value2.Get(context) +
                   Value3.Get(context);
        }
    }
}

With this new activity you can rewrite the previous workflow that added three values as shown in the following example:

Variable a = new Variable("a", 1);
Variable b = new Variable("b", 2);
Variable c = new Variable("c", 3);
Variable r = new Variable("r", 0);

Sequence w = new Sequence
{
    Variables = { a, b, c, r },
    Activities =
    {
        new Assign {
            To = new OutArgument(r),
            Value = new InArgument {
                Expression = new AddThreeValues() {
                    Value1 = new InArgument(a),
                    Value2 = new InArgument(b),
                    Value3 = new InArgument(c)
                }
            }
        }
    }
};

For more information about using expressions in code, see Authoring Workflows, Activities, and Expressions Using Imperative Code.