生成表达式以筛选数据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;

我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

我终于找到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关于禁止客户端评估的回应，我能够验证过滤没有在客户端上执行

Answer 2

配置DbContext以禁止客户端评估和测试。

例如

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

请参阅https://learn.microsoft.com/en-us/ef/core/querying/client-eval#previous-版本

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"功能"。