//----------------------------------------------------------------
// Copyright (c) Microsoft Corporation.  All rights reserved.
//----------------------------------------------------------------

namespace System.Activities.Expressions
{
    using System.Activities;
    using System.Activities.Statements;
    using System.Activities.Validation;
    using System.Collections.Generic;
    using System.ComponentModel;
    using System.Diagnostics.CodeAnalysis;
    using System.Linq.Expressions;
    using System.Runtime;

    [SuppressMessage(FxCop.Category.Naming, FxCop.Rule.IdentifiersShouldNotMatchKeywords,
        Justification = "Optimizing for XAML naming. VB imperative users will [] qualify (e.g. New [As])")]
    public sealed class As<TOperand, TResult> : CodeActivity<TResult>
    {
        //Lock is not needed for operationFunction here. The reason is that delegates for a given As<TLeft, TRight, TResult> are the same.
        //It's possible that 2 threads are assigning the operationFucntion at the same time. But it's okay because the compiled codes are the same.
        static Func<TOperand, TResult> operationFunction;

        [RequiredArgument]
        [DefaultValue(null)]
        public InArgument<TOperand> Operand
        {
            get;
            set;
        }

        protected override void CacheMetadata(CodeActivityMetadata metadata)
        {
            UnaryExpressionHelper.OnGetArguments(metadata, this.Operand);

            if (operationFunction == null)
            {
                ValidationError validationError;
                if (!UnaryExpressionHelper.TryGenerateLinqDelegate(ExpressionType.TypeAs, out operationFunction, out validationError))
                {
                    metadata.AddValidationError(validationError);
                }
            }
        }

        protected override TResult Execute(CodeActivityContext context)
        {
            Fx.Assert(operationFunction != null, "OperationFunction must exist.");
            TOperand operandValue = this.Operand.Get(context);
            return operationFunction(operandValue);
        }
    }
}