用表达式实现LINQ筛选器

本文关键字：筛选 LINQ 实现 表达式 | 更新日期: 2023-09-27 18:24:47

在MVC4中，我为用户提供了搜索框，用于搜索表中的任何值。所以我在C#中在服务器端实现通用过滤条件

需要帮助组合多个表达式以形成单个表达式

 Expression<Func<T, bool>>

例如

表格列

MenuText，角色名称（角色名称映射），ActionName

现在，如果用户在搜索框中输入ABC，则需要进行筛选，ABC可以在所示列中的任何行中。

型号

public class Menu
{
  public string MenuText {get;set;}
  public Role Role {get;set;}
  public string ActionName {get;set;}
}
public class Role
{
  public string Name {get;set;}
}

到目前为止，我已经实现了

  /// <summary>
    /// string[] properties property.Name (MenuText, ActionName), including deeper Mapping names such as (Role.Name)
    /// </summary>
    public static Expression<Func<T, bool>> FilterKey<T>(string filterText, params string[] properties)
    {
        ParameterExpression parameter = Expression.Parameter(typeof (T));
        Expression[] propertyExpressions = properties.Select(
            x => !string.IsNullOrEmpty(x) ? GetDeepPropertyExpression(parameter, x) : null).ToArray();
        Expression<Func<T, bool>> predicate = PredicateBuilder.False<T>();
        foreach (Expression expression in propertyExpressions)
        {
             var toLower = Expression.Call(expression, typeof(string).GetMethod("ToLower", System.Type.EmptyTypes));
            var like = Expression.Call(toLower, typeof(string).GetMethod("Contains"), Expression.Constant(filterText.ToLower()));
            //TODO: Combine expressions to form single  Expression<Func<T, bool>> expression
        }
        return predicate;
    }
       /// <summary>
        ///  To Get Deeper Properties such as Role.Name Expressions
        /// </summary>
       private static Expression GetDeepPropertyExpression(Expression initialInstance, string property)
        {
            Expression result = null;
            foreach (string propertyName in property.Split('.'))
            {
                Expression instance = result ?? initialInstance;
                result = Expression.Property(instance, propertyName);
            }
            return result;
        }

用表达式实现LINQ筛选器

我创建了一些搜索IQueryable扩展方法，您应该能够使用

完整的博客文章在这里：

http://jnye.co/Posts/6/c%23-可查询的通用搜索扩展方法

GitHub项目就在这里（有几个额外的OR搜索扩展：

https://github.com/ninjanye/SearchExtensions

public static class QueryableExtensions  
{  
    public static IQueryable<T> Search<T>(this IQueryable<T> source, Expression<Func<T, string>> stringProperty, string searchTerm)  
    {  
        if (String.IsNullOrEmpty(searchTerm))  
        {  
            return source;  
        }  
        // The below represents the following lamda:  
        // source.Where(x => x.[property] != null  
        //                && x.[property].Contains(searchTerm))  
        //Create expression to represent x.[property] != null  
        var isNotNullExpression = Expression.NotEqual(stringProperty.Body, Expression.Constant(null));  
        //Create expression to represent x.[property].Contains(searchTerm)  
        var searchTermExpression = Expression.Constant(searchTerm);  
        var checkContainsExpression = Expression.Call(stringProperty.Body, typeof(string).GetMethod("Contains"), searchTermExpression);  
        //Join not null and contains expressions  
        var notNullAndContainsExpression = Expression.AndAlso(isNotNullExpression, checkContainsExpression);  
        var methodCallExpression = Expression.Call(typeof(Queryable),  
                                                   "Where",  
                                                   new Type[] { source.ElementType },  
                                                   source.Expression,  
                                                   Expression.Lambda<Func<T, bool>>(notNullAndContainsExpression, stringProperty.Parameters));  
        return source.Provider.CreateQuery<T>(methodCallExpression);  
    }  
}

这允许你写一些类似的东西：

string searchTerm = "test";  
var results = context.Menu.Search(menu => menu.MenuText, searchTerm).ToList();  
//OR for Role name
string searchTerm = "test";
var results = context.Menu.Search(menu => menu.Role.Name, searchTerm).ToList();

你可能还会发现以下帖子很有用：

允许搜索accros多个属性的搜索扩展方法：

http://jnye.co/Posts/7/generic-iqueryable-or-search-on-multiple-properties-using-expression-trees

允许在一处房产上使用多个或搜索词的搜索扩展方法：

http://jnye.co/Posts/8/generic-iqueryable-or-search-for-multiple-search-terms-using-expression-trees

感谢NinjaNye，我借用了BuildOrExpression解决了问题

这是的解决方案

   public static Expression<Func<T, bool>> FilterKey<T>(string filterText, params string[] properties)
        {
            ParameterExpression parameter = Expression.Parameter(typeof (T));
            Expression[] propertyExpressions = properties.Select(
                x => !string.IsNullOrEmpty(x) ? GetDeepPropertyExpression(parameter, x) : null).ToArray();
            Expression like= propertyExpressions.Select(expression => Expression.Call(expression, typeof (string).GetMethod("ToLower", Type.EmptyTypes))).Select(toLower => Expression.Call(toLower, typeof (string).GetMethod("Contains"), Expression.Constant(filterText.ToLower()))).Aggregate<MethodCallExpression, Expression>(null, (current, ex) => BuildOrExpression(current, ex));
            return  Expression.Lambda<Func<T, bool>>(like, parameter);
        }
        private static Expression BuildOrExpression(Expression existingExpression, Expression expressionToAdd)
        {
            if (existingExpression == null)
            {
                return expressionToAdd;
            }
            //Build 'OR' expression for each property
            return Expression.OrElse(existingExpression, expressionToAdd);
        }

       private static Expression GetDeepPropertyExpression(Expression initialInstance, string property)
        {
            Expression result = null;
            foreach (string propertyName in property.Split('.'))
            {
                Expression instance = result ?? initialInstance;
                result = Expression.Property(instance, propertyName);
            }
            return result;
        }
相关文章：