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，对吧？

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

Answer 1

配置您的 DbContext 以禁止客户端求值和测试。

例如

protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
    optionsBuilder
        .UseSqlServer(@"Server=(localdb)\mssqllocaldb;Database=EFQuerying;Trusted_Connection=True;")
        .ConfigureWarnings(warnings => warnings.Throw(RelationalEventId.QueryClientEvaluationWarning));
}

见https://docs.microsoft.com/en-us/ef/core/querying/client-eval#previous-versions

或者开始迁移到 EF Core 3.1，它增强了查询翻译功能，并删除了客户端评估 "feature"。

Answer 2

我终于弄明白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();

一般示例：

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 关于的回复，我能够验证过滤没有在客户端执行

构建表达式以过滤数据 EF Core

Build Expression to filter data EF Core

c#

entity-framework

linq-expressions

ef-core-2.2