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c# - 可查询的高级 where 的扩展方法

问题描述

我正在尝试添加扩展方法IQueryable<T>，以便将特定的位置附加到IQueryable.

假设用户正在搜索名称等于的数据"Matt"

public IEnumerable<Employees> Search(String name)
{
    var qList = _context.Employees;
    if(!String.isNullOrEmpty(name))
        qList = qList.Where(emp => emp.Name == name);
    return qList.ToList();
}

现在，如果用户正在搜索名称以"Matt"- 开头的数据，他可能会尝试在特定的文本框中写一些"Matt%**"东西。"Matt*"为了预测这一点，我可以做到：

public IEnumerable<Employees> Search(String name)
{
    var qList = _context.Employees;
    if(!String.isNullOrEmpty(name))
    {
        string extractedName = name.Replace("%","").Replace("*","");
        if(name.EndsWith("%") || name.EndsWith("*");
            qList = qList.Where(emp => emp.Name.StartsWith(extractedName));
    }

    return qList.ToList();
}

很多 IF 语句将在这里预测所有可能性，但没关系。我能做到。但我不喜欢每次执行此类功能时都重复我的代码。

我如何制作扩展方法

IQueryable<T>

哪个会一直为我做这个检查？

public static IQueryable<T> MyAdvancedSearch<T>(this IQueryable<T> qList, String searchText)
{
    if(!String.isNullOrEmpty(searchText))
    /// ... Problem starts here.. because I cannot make this "Where": 
    qList.Where(prop=>prop.??? == searchText);
    /// ... I dont know how to tell my method which field should be filtered
}

更新 我设法创建了一个看起来不错的扩展方法：

        public static IQueryable<T> Filter<T>(this IQueryable<T> qList, Func<T,string> property, string text )
        {
            if (!String.IsNullOrEmpty(text))
            {
                if ((text.StartsWith("%") || text.StartsWith("*")) && (text.EndsWith("*") || text.EndsWith("*")))
                    qList = qList.Where(e => property(e).Contains(text));

                else if (text.StartsWith("%") || text.StartsWith("*"))
                    qList = qList.Where(e => property(e).EndsWith(text));

                else if (text.EndsWith("%") || text.EndsWith("*"))
                    qList = qList.Where(e => property(e).StartsWith(text));
                else
                    qList = qList.Where(e => property(e) == text);
            }
            return qList;
        }

但是当我尝试做 qList.Tolist() 我收到错误：

InvalidOperationException: The LINQ expression 'DbSet<Employee>()
.Where(o => Invoke(__property_0, o)
== __text_1)' could not be translated.

InvalidOperationException: The LINQ expression 'DbSet<Employee>() .Where(o => Invoke(__property_0, o) == __text_1)' 
could not be translated. Either rewrite the query in a form that can be translated, or switch to client evaluation explicitly by inserting a call to 'AsEnumerable', 'AsAsyncEnumerable', 'ToList', or 'ToListAsync'. 
See https://go.microsoft.com/fwlink/?linkid=2101038 for more information.

标签： c#linqextension-methods

解决方案

这应该可以解决问题

public static IQueryable<T> MyAdvancedSearch<T>(this IQueryable<T> qList, Expression<Func<T, string>> property, String text)
{
    Expression<Func<T, bool>> expression = null;

    var propertyName = GetPropertyName(property);
    var parameter = Expression.Parameter(typeof(T), "x");
    var left = propertyName.Split('.').Aggregate((Expression)parameter, Expression.Property);
    var right = Expression.Constant(text, typeof(string));
    Expression body = null;

    if (!string.IsNullOrEmpty(text))
    {
        if ((text.StartsWith("%") || text.StartsWith("*")) && (text.EndsWith("%") || text.EndsWith("*")))
            body = Expression.Call(left, "Contains", Type.EmptyTypes, Expression.Constant(text, typeof(string)));

        else if (text.StartsWith("%") || text.StartsWith("*"))
                body = Expression.Call(left, "EndsWith", Type.EmptyTypes, Expression.Constant(text, typeof(string)));

        else if (text.EndsWith("%") || text.EndsWith("*"))
                body = Expression.Call(left, "StartsWith", Type.EmptyTypes, Expression.Constant(text, typeof(string)));
        else
                body = Expression.MakeBinary(ExpressionType.Equal, left, right);
    }

    expression = Expression.Lambda<Func<T, bool>>(body, parameter);

    qList = qList.Where(expression);
        return qList;
}
public static string GetPropertyName<TClass, TProperty>(Expression<Func<TClass, TProperty>> property)
{
    MemberExpression member = property.Body as MemberExpression;
    PropertyInfo info = member.Member as PropertyInfo;

    return info.Name;
}

public static IQueryable<T> MyAdvancedSearch<T>(this IQueryable<T> qList, Func<T, string> func, String searchText)
        {
            qList.Where(prop => func(prop) == searchText);

            return qList;
        }
然后你可以这样称呼它： 
var qList = _context.Employees.MyAdvancedSearch((emp) => emp.Name, extractedname)

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