在 LinqToEntities 中，如何将动态列名传递给 DbFunctions.Like答案

【问题标题】：In LinqToEntities, How to pass dynamic column name to DbFunctions.Like在 LinqToEntities 中，如何将动态列名传递给 DbFunctions.Like
【发布时间】：2020-08-01 06:28:54
【问题描述】：

我在 Entity Framework 中有一个来自 DbSet 的 IQueryable<T>。为我提供了一个名为 searchText 的“模糊搜索字符串”，如下所示：

public List<T> Search<T>(string searchText)
{
    using (var context = ...)
    {
        var baseQuery = context.Set<T>().AsQueryable();
        baseQuery = baseQuery.Where(x =>
            DbFunctions.Like(x.PropertyName, searchText)
            || DbFunctions.Like(x.PropertyTwo, searchText)
            || DbFunctions.Like(x.PropertyThree, searchText)
            || DbFunctio..... etc
        );
        return baseQuery.ToList();
    }
}

但鉴于通用性质，我不知道该类型有哪些属性。我可以为实现它的人提供一个抽象方法，使他们可以给我一个属性列表（甚至PropertyInfo 或其他任何东西，我可以弄清楚）。但我不知道如何动态创建表达式。这是我目前所拥有的：

var baseQuery = context.Set<T>().AsQueryable();
var expression = baseQuery.Expression;
var colName = "colName"; // Or names, I can iterate.

var parameter = Expression.Parameter(typeof(T), "x");
var selector = Expression.PropertyOrField(parameter, colName);
expression = Expression.Call(typeof(DbFunctions), nameof(DbFunctions.Like),
    new Type[] { baseQuery.ElementType, selector.Type },
    expression, Expression.Quote(Expression.Lambda(selector, parameter)));

这里的问题是……好吧，它一开始是行不通的。但主要是我没有在其中的任何地方使用searchText，也不知道如何插入它。我想我已经接近了......但在它上面花费了过多的时间。

【问题讨论】：

标签： c# entity-framework-6 linq-to-entities iqueryable linq-expressions

【解决方案1】：

希望我的查询逻辑正确：如果您想根据已知类型和列名列表构建一组 LIKE 条件，您可以尝试这样的操作：

static private MethodInfo dbLikeMethod = typeof(DbFunctions).GetMethod(nameof(DbFunctions.Like), BindingFlags.Public | BindingFlags.Static, null, new Type[] {typeof(string), typeof(string)}, null); // I am targeting DbFunctions.Like(string, string). You might want another overload (or even mix them up depending on your inputs)

public List<T> Search<T>(string searchText) where T: class
{

    using (var context = new ...)
    {
        var baseQuery = context.Set<T>().AsQueryable().Where(CreateExpression<T>(searchText));// you could probably find a more elegant way of plugging it into your query
        return baseQuery.ToList();
    }
}

Expression<Func<T, bool>> CreateExpression<T>(string searchText) where T : class
{   
    var cols = new List<string> {
        "PropertyName",
        "PropertyTwo" // i understand you've got a way to figure out which strings you need here
    };

    var parameter = Expression.Parameter(typeof(T), "x");   
    var dbLikeCalls = cols.Select(colName => Expression.Call(dbLikeMethod, Expression.PropertyOrField(parameter, colName), Expression.Constant(searchText))); // for convenience, generate list of DbFunctions.Like(x.<Property>, searchText) expressions here
    var aggregatedCalls = dbLikeCalls.Skip(1).Aggregate((Expression)dbLikeCalls.First(), (accumulate, call) => Expression.OrElse(accumulate, call)); // aggregate the list using || operators: use first item as a seed and keep adding onto it

    return Expression.Lambda<Func<T, bool>>(aggregatedCalls, parameter);
}

【讨论】：