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IRuleExpression Interface

Represents the base class from which custom expression writers must derive to write custom expressions.

Namespace:  System.Workflow.Activities.Rules
Assembly:  System.Workflow.Activities (in System.Workflow.Activities.dll)
public interface IRuleExpression

The IRuleExpression type exposes the following members.

Public methodAnalyzeUsageWhen overridden in a derived class, reports on how the object uses fields and properties in the context type.
Public methodCloneWhen overridden in a derived class, creates a deep copy of the current CodeExpression.
Public methodDecompileWhen overridden in a derived class, decompiles the custom expression into string form.
Public methodEvaluateWhen overridden in a derived class, evaluates the custom expression.
Public methodMatchCompares the current expression to another expression to determine whether they are equal.
Public methodValidateWhen overridden in a derived class, verifies that the expression is configured correctly and has no errors.

The following code creates an expression that can be used in declarative conditions and rule sets. The expression is named TwoOfThree, and takes 3 parameters, all of which must evaluate to Booleans. This expression returns true if 2 of the 3 expressions return true.

To use this code, add it to a Class Library project and reference the library from your workflow project.

using System.CodeDom;
using System.Text;
using System.Workflow.Activities.Rules;
using System.Workflow.ComponentModel.Compiler;

namespace TwoOfThreeRuleExpression
    public class TwoOfThree : CodeExpression, IRuleExpression
        CodeExpression expression1, expression2, expression3;

        public CodeExpression First
            get { return expression1; }
            set { expression1 = value; }

        public CodeExpression Second
            get { return expression2; }
            set { expression2 = value; }

        public CodeExpression Third
            get { return expression3; }
            set { expression3 = value; }

        public TwoOfThree()
            // constructor required for deserialization

        public TwoOfThree(CodeExpression first, CodeExpression second, CodeExpression third)
            // constructor required by parser
            expression1 = first;
            expression2 = second;
            expression3 = third;

        public void AnalyzeUsage(RuleAnalysis analysis, bool isRead, bool isWritten, RulePathQualifier qualifier)
            // check what the 3 expressions use
            RuleExpressionWalker.AnalyzeUsage(analysis, expression1, true, false, null);
            RuleExpressionWalker.AnalyzeUsage(analysis, expression2, true, false, null);
            RuleExpressionWalker.AnalyzeUsage(analysis, expression3, true, false, null);

        public CodeExpression Clone()
            TwoOfThree result = new TwoOfThree();
            result.expression1 = RuleExpressionWalker.Clone(expression1);
            result.expression2 = RuleExpressionWalker.Clone(expression2);
            result.expression3 = RuleExpressionWalker.Clone(expression3);
            return result;

        public void Decompile(StringBuilder stringBuilder, CodeExpression parentExpression)
            // what should be displayed by the parser
            RuleExpressionWalker.Decompile(stringBuilder, expression1, this);
            stringBuilder.Append(", ");
            RuleExpressionWalker.Decompile(stringBuilder, expression2, this);
            stringBuilder.Append(", ");
            RuleExpressionWalker.Decompile(stringBuilder, expression3, this);

        static RuleLiteralResult resultTrue = new RuleLiteralResult(true);
        static RuleLiteralResult resultFalse = new RuleLiteralResult(false);

        public RuleExpressionResult Evaluate(RuleExecution execution)
            // start by doing the first 2 expressions
            RuleExpressionResult r1 = RuleExpressionWalker.Evaluate(execution, expression1);
            RuleExpressionResult r2 = RuleExpressionWalker.Evaluate(execution, expression2);
            bool b1 = (bool)r1.Value;
            bool b2 = (bool)r2.Value;
            if (b1 && b2)
                // both are true, so result is true
                return resultTrue;
            else if (b1 || b2)
                // only one of the first 2 is true, evaluate the third to determine result
                return RuleExpressionWalker.Evaluate(execution, expression3);
                // both e1 and e2 are false, so skip e3 and return false;
                return resultFalse;

        public bool Match(CodeExpression expression)
            TwoOfThree other = expression as TwoOfThree;
            return (other != null) &&
                RuleExpressionWalker.Match(expression1, other.expression1) &&
                RuleExpressionWalker.Match(expression2, other.expression2) &&
                RuleExpressionWalker.Match(expression3, other.expression3);

        public RuleExpressionInfo Validate(RuleValidation validation, bool isWritten)
            ValidateExpression(validation, expression1, "First");
            ValidateExpression(validation, expression2, "Second");
            ValidateExpression(validation, expression3, "Third");
            return new RuleExpressionInfo(typeof(bool));

        private void ValidateExpression(RuleValidation validation, CodeExpression expression, string propertyName)
            ValidationError error;
            if (expression == null)
                error = new ValidationError(propertyName + " cannot be null", 123);
                RuleExpressionInfo result = RuleExpressionWalker.Validate(validation, expression, false);
                if ((result == null) || (result.ExpressionType != typeof(bool)))
                    error = new ValidationError(propertyName + " must return boolean result", 123);

.NET Framework

Supported in: 4.5.1, 4.5, 4, 3.5, 3.0

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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