c# - 在运行时创建 lambda 表达式

Question

我有一个存储库类，它有一个定义如下的 GetAsQueryable 方法:

public class Repository<TEntity> : IDisposable, IRepository<TEntity> where TEntity : class
{
    internal DbSet<TEntity> _DbSet;

    public virtual IQueryable<TEntity> GetAsQueryable(
         Expression<Func<TEntity, bool>> filter = null,
         Func<IQueryable<TEntity>, IOrderedQueryable<TEntity>> orderBy = null,
         string includeProperties = "")
    {
        IQueryable<TEntity> query = _DbSet;

        if (filter != null)
        {
            query = query.Where(filter);
        }

        foreach (var includeProperty in includeProperties.Split
            (new char[] { ',' }, StringSplitOptions.RemoveEmptyEntries))
        {
            query = query.Include(includeProperty);
        }

        if (orderBy != null)
        {
            return orderBy(query);
        }
        else
        {
            return query;
        }
    }
}

在我的调用代码中，我通常会执行以下操作:

IRepository<Tracking> repoTracking = new Repository<Tracking>(context);
IQueryable<Tracking> tracking = repoTracking.GetAsQueryable();
var results = tracking.Where(t => t.Status_Code_Id == 15).ToList();

这很好用；但是现在我希望能够构建在运行时发送到 .Where 的 lambda。我试过构建一个 expression tree如下:

IRepository<Tracking> repoTracking = new Repository<Tracking>(context);
IQueryable<Tracking> tracking = repoTracking.GetAsQueryable();
var results = tracking.Where(t => t.Status_Code_Id == 15).ToList();

IRepository<Tracking> repoTracking = new Repository<Tracking>(context);
IQueryable<Tracking> tracking = repoTracking.GetAsQueryable();

ParameterExpression pe = Expression.Parameter(typeof (int), "Status_Code_Id");
LambdaExpression lambda = Expression.Lambda(Expression.Equal(pe, Expression.Constant(15)));
MethodCallExpression whereExpression = Expression.Call(
    typeof(Queryable),
    "Where",
    new Type[] { tracking.ElementType },
    tracking.Expression,
    lambda);

但是，这会产生以下异常:

No generic method 'Where' on type 'System.Linq.Queryable' is compatible with the supplied type arguments and arguments. No type arguments should be provided if the method is non-generic.

为什么在我的 Tracking 实例上找不到 Where 方法？

Answer 1

如果你想构建t => t.Status_Code_Id == 15动态你不需要调用Where使用 Expression .只需创建 Expression<Func<TEntity, bool>>并将其传递给 Where :

ParameterExpression t = Expression.Parameter(typeof(Tracking), "t");
Expression statusCode = Expression.Property(t, "Status_Code_Id");
Expression comparison = Expression.Equal(statusCode, Expression.Constant(15));
Expression<Func<Tracking, bool>> lambda
    = Expression.Lambda<Func<Tracking, bool>>(comparison, t);

var results = tracking.Where(lambda).ToList();