为 EfCore 构建谓词 IQueryable.Where<T>

Question

我想为 IQueryable 创建一个 Search<T> 扩展方法，但是遇到了一个无法翻译的 LINQ 表达式运行时错误。

public static IQueryable<T> Search<T>(this IQueryable<T> source, Func<T, string> expression, string search)
{
    if (source == null)
        throw new ArgumentNullException(nameof(source));

    if (string.IsNullOrWhiteSpace(search))
        return source;

    return source.Where(x => EF.Functions.Like(expression(x), $"%{search}%"));
}

用法：

dbContext.People.Search(x => x.Name, "Joe").ToList();

我猜错误的行是expression(x)，但正确的方法是什么？

Answer 1

它不使用 Like 功能，但作为扩展可以正常工作。

var result = dbContex.Currencies.Search(x=>x.Symbol, "EUR").ToList();


public static IQueryable<T> Search<T>(this IQueryable<T> source,  Expression<Func<T,string>> expression, string search)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));

            if (string.IsNullOrWhiteSpace(search))
                return source;
            var method = typeof(string).GetMethod("Contains", new[] { typeof(string) });
            var value = Expression.Constant(search, typeof(string));
            var exp1 = Expression.Call(expression.Body, method, value);
            var ex2 = Expression.Lambda<Func<T, bool>>(exp1, expression.Parameters[0]);
            

            return source.Where(ex2);
        }

Answer 2

这是一个经过测试且有效的解决方案。

public static IQueryable<T> Search<T>(this IQueryable<T> source, Expression<Func<T, string>> expression, string search)
{
    if (source == null)
        throw new ArgumentNullException(nameof(source));

    if (string.IsNullOrWhiteSpace(search))
        return source;

    var property = typeof(T).GetProperty(((MemberExpression)expression.Body).Member.Name);
    var parameter = Expression.Parameter(typeof(T));
    var functions = Expression.Property(null, typeof(EF).GetProperty(nameof(EF.Functions)));
    var like = typeof(DbFunctionsExtensions).GetMethod(nameof(DbFunctionsExtensions.Like),
        new[] { functions.Type, typeof(string), typeof(string) });

    Expression expressionProperty = Expression.Property(parameter, property.Name);

    var body = Expression.Call(null, like, functions, expressionProperty, Expression.Constant($"%{search}%"));

    return source.Where(Expression.Lambda<Func<T, bool>>(body, parameter));
}

为 EfCore 构建谓词 IQueryable.Where<T>

Build predicate for EfCore IQueryable.Where<T>

.net

c#

ef-core-3.1