//这里必须转换，否则如果字段可为NULL，就会报错

schangxiang@126.com

2025-05-13 14079a3004cf9df7b9bd77aff48aea866b84e169

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using System;
using System.ComponentModel;
using System.Linq.Expressions;
using System.Reflection;
 
namespace CmsQueryExtensions.Extension
{
    public class WhereHelper<T>
 
    //where T : class
 
    {
 
        public ParameterExpression param;
 
        public BinaryExpression filter;
 
        private MemberExpression common_left;
 
        private UnaryExpression common_right;
        //private ConstantExpression common_right;
 
        public WhereHelper()
 
        {
 
            param = Expression.Parameter(typeof(T), "c");//构建 c=> 结构
 
            //1==1
            Expression left = Expression.Constant(1);
            filter = Expression.Equal(left, left);//构建出 c=> 1=1
 
        }
 
        public Expression<Func<T, bool>> GetExpression()
 
        {
 
            return Expression.Lambda<Func<T, bool>>(filter, param);
 
        }
 
        private void CommonLeftRight(string propertyName, object value)
        {
 
            common_left = Expression.Property(param, typeof(T).GetProperty(propertyName));//构建构建c.{propertyName}的结构
 
            var member = Expression.Property(param, propertyName);
            var propertyType = ((PropertyInfo)member.Member).PropertyType;
            var converter = TypeDescriptor.GetConverter(propertyType); // 1
 
            if (!converter.CanConvertFrom(typeof(string))) // 2
                throw new NotSupportedException();
 
            var propertyValue = converter.ConvertFromInvariantString(value.ToString()); // 3
            var constant = Expression.Constant(propertyValue);
            //common_right = constant; // 4
            common_right = Expression.Convert(constant, propertyType); // 4 //这里必须转换，否则如果字段可为NULL，就会报错
            // common_right = Expression.Constant(value, value.GetType());//构建一个常量,值是  value
        }
 
        /// <summary>
        /// 等于 =
        /// </summary>
        /// <param name="propertyName"></param>
        /// <param name="value"></param>
        public void Equal(string propertyName, object value)
        {
            CommonLeftRight(propertyName, value);
 
            Expression result = Expression.Equal(common_left, common_right);
 
            filter = Expression.AndAlso(filter, result);
        }
 
        /// <summary>
        /// 不等于 <>
        /// </summary>
        /// <param name="propertyName"></param>
        /// <param name="value"></param>
        public void NotEqual(string propertyName, object value)
        {
 
            CommonLeftRight(propertyName, value);
 
            Expression result = Expression.NotEqual(common_left, common_right);
 
            filter = Expression.AndAlso(filter, result);
 
        }
 
        /// <summary>
        /// 大于 >
        /// </summary>
        /// <param name="propertyName"></param>
        /// <param name="value"></param>
        public void GreaterThan(string propertyName, object value)
        {
 
            CommonLeftRight(propertyName, value);
 
            Expression result = Expression.GreaterThan(common_left, common_right);
 
            filter = Expression.AndAlso(filter, result);
 
        }
 
        /// <summary>
        /// 大于等于 >=
        /// </summary>
        /// <param name="propertyName"></param>
        /// <param name="value"></param>
        public void GreaterThanOrEqual(string propertyName, object value)
        {
 
            CommonLeftRight(propertyName, value);
 
            Expression result = Expression.GreaterThanOrEqual(common_left, common_right);
 
            filter = Expression.AndAlso(filter, result);
 
        }
 
        /// <summary>
        /// 小于 <
        /// </summary>
        /// <param name="propertyName"></param>
        /// <param name="value"></param>
        public void LessThan(string propertyName, object value)
        {
 
            CommonLeftRight(propertyName, value);
 
            Expression result = Expression.LessThan(common_left, common_right);
 
            filter = Expression.AndAlso(filter, result);
 
        }
 
        /// <summary>
        /// 小于等于 <=
        /// </summary>
        /// <param name="propertyName"></param>
        /// <param name="value"></param>
        public void LessThanOrEqual(string propertyName, object value)
        {
 
            CommonLeftRight(propertyName, value);
 
            Expression result = Expression.LessThanOrEqual(common_left, common_right);
 
            filter = Expression.AndAlso(filter, result);
 
        }
 
        /// <summary>
        /// 包含 LIKE
        /// </summary>
        /// <param name="propertyName"></param>
        /// <param name="value"></param>
        public void Contains(string propertyName, string value)
 
        {
 
            Expression left = Expression.Property(param, typeof(T).GetProperty(propertyName));
 
            Expression right = Expression.Constant(value, value.GetType());
 
            Expression result = Expression.Call(left, typeof(string).GetMethod("Contains", new Type[] { typeof(string) }), right);
 
            filter = Expression.AndAlso(filter, result);
 
        }
 
 
 
    }
}