c# - 构建表达式以过滤数据 EF Core

Question

我需要重用可用的表达式:

Expression<Func<Picture, int>> selector = o => o.EntityId;

并为 Where 构建表达式:

Expression<Func<Picture, bool>> filter = w => w.EntityId > 5;

如何构建这样的表达式？

下一个操作不会在客户端执行，对吗？

var collection = _dbContext.Pictures.Where(filter).ToList();

Answer 1

我终于想出了如何动态构建表达式:

Expression<Func<Picture, int>> selector = o => o.EntityId;

var parameter = Expression.Parameter(typeof(Picture));

// get property name
if (!(selector.Body is MemberExpression memberExpression))
{
    memberExpression = ((UnaryExpression)selector.Body).Operand as MemberExpression;
}
var propertyName = memberExpression.ToString().Substring(2);

var expressionParameter = Expression.Property(parameter, propertyName);
var expressionBody = Expression.GreaterThan(expressionParameter, Expression.Constant(5, typeof(int)));

var filter = Expression.Lambda<Func<Picture, bool>>(expressionBody, parameter);
var collection = _dbContext.Pictures.Where(filter).ToList();

通用示例:

var filter = CreateFilter<Picture, int>(o => o.EntityId, 5);
var collection = _dbContext.Pictures.Where(filter).ToList();

private Expression<Func<TData, bool>> CreateFilter<TData, TKey>(Expression<Func<TData, TKey>> selector, TKey valueToCompare)
{
    var parameter = Expression.Parameter(typeof(TData));
    var expressionParameter = Expression.Property(parameter, GetParameterName(selector));

    var body = Expression.GreaterThan(expressionParameter, Expression.Constant(valueToCompare, typeof(TKey)));
    return Expression.Lambda<Func<TData, bool>>(body, parameter);
}

private string GetParameterName<TData, TKey>(Expression<Func<TData, TKey>> expression)
{
    if (!(expression.Body is MemberExpression memberExpression))
    {
        memberExpression = ((UnaryExpression)expression.Body).Operand as MemberExpression;
    }

    return memberExpression.ToString().Substring(2);
}

感谢 David 关于 Prohibit client-side evaluation 的回复, 我能够验证过滤没有在客户端上执行