using System;
using System.Linq.Expressions;

namespace Util.Lambdas {
    /// <summary>
    /// 表達式生成器
    /// </summary>
    public class ExpressionBuilder<TEntity> {
        /// <summary>
        /// 初始化表達式生成器
        /// </summary>
        public ExpressionBuilder() {
            Parameter = Expression.Parameter( typeof( TEntity ), "t" );
        }

        /// <summary>
        /// 參數
        /// </summary>
        private ParameterExpression Parameter { get; set; }

        /// <summary>
        /// 獲取參數
        /// </summary>
        public ParameterExpression GetParameter() {
            return Parameter;
        }

        /// <summary>
        /// 創建表達式
        /// </summary>
        /// <param name="property">屬性表達式</param>
        /// <param name="operator">運算符</param>
        /// <param name="value">值</param>
        public Expression Create<T>( Expression<Func<TEntity, T>> property, Operator @operator, object value ) {
            return Parameter.Property( Lambda.GetMember( property ) ).Operation( @operator, value );
        }

        /// <summary>
        /// 轉換為Lambda表達式
        /// </summary>
        /// <param name="expression">表達式</param>
        public Expression<Func<TEntity, bool>> ToLambda( Expression expression ) {
            if ( expression == null )
                return null;
            return expression.ToLambda<Func<TEntity, bool>>( Parameter );
        }
    }
}

using System;
using System.Linq.Expressions;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using Util.Lambdas;
using Util.Tests.Samples;

namespace Util.Tests {
    /// <summary>
    /// 測試表達式生成器
    /// </summary>
    [TestClass]
    public class ExpressionBuilderTest {
        /// <summary>
        /// 表達式生成器
        /// </summary>
        ExpressionBuilder<Test2> _builder;

        /// <summary>
        /// 測試初始化
        /// </summary>
        [TestInitialize]
        public void TestInit() {
            _builder = new ExpressionBuilder<Test2>();
        }

        /// <summary>
        /// 創建表達式
        /// </summary>
        [TestMethod]
        public void TestCreate_Int() {
            Expression<Func<Test2, int>> property = t => t.Int;
            var expression = _builder.Create( property, Operator.Equal, 1 );
            Expression<Func<Test2, bool>> expected = t => t.Int == 1;
            Assert.AreEqual( expected.ToString(), _builder.ToLambda( expression ).ToString() );
        }

        /// <summary>
        /// 創建表達式
        /// </summary>
        [TestMethod]
        public void TestCreate_Int_Nullable() {
            Expression<Func<Test2, int?>> property = t => t.NullableInt;
            var expression = _builder.Create( property, Operator.Equal, 1 );
            Assert.AreEqual( "t => (t.NullableInt == 1)", _builder.ToLambda( expression ).ToString() );
        }
    }
}