内部带有另一个 lambda 的 Lambda IfThenElse 表达式（用于 SetQueryFilter()）

Question

我的目标是对IMutableEntityType 进行过滤，我通过CurrentTenantId 列值选择数据行。 更多上下文请见：https://github.com/dotnet/efcore/issues/23718

这已经是我做的了：

var parameter = Expression.Parameter(entityType.ClrType, "p");
                
// TODO: This filter is working, but we must prevent filtering with TenantId = 0
var workingFilter = Expression.Lambda(
    Expression.Equal(
        Expression.Property(parameter, colName),
        // See https://github.com/dotnet/efcore/issues/23718
        Expression.MakeMemberAccess(Expression.Constant(myContext), currentTenantIdMemberInfo)
        ),
    parameter);

entityType.SetQueryFilter(filter);

但是如果TenantId == 0我想获取所有实体，所以我写了这个Lambda：

var conditionalExpression = Expression.IfThenElse(
    // Check if TenantId > 0
    Expression.GreaterThan(
        Expression.MakeMemberAccess(Expression.Constant(myContext), currentTenantIdMemberInfo),
        Expression.Constant((long) 0)
        ),
    // build filter: (p) => p.{colName} == {myContext.CurrentTenantId}
    // i.e. (p) => p.TenantId == 123)
    Expression.Lambda(
        Expression.Equal(
            Expression.Property(parameter, colName),

            // See https://github.com/dotnet/efcore/issues/23718
            Expression.MakeMemberAccess(Expression.Constant(myContext), currentTenantIdMemberInfo)
            )//,
        ),
    // TenantId <= 0, so act like (p => p)
    Expression.Lambda(Expression.Constant(parameter)) // just p // TODO: Not working
    //Expression.Lambda(Expression.Constant(parameter), parameter) // p => p // TODO: Also not working...
);

var filter = Expression.Lambda(conditionalExpression);

但是在调用entityType.SetQueryFilter(filter);之后我有InvalidOperationException：

'The filter expression '() => IIF((value(My.Project.MyContext).CurrentTenantId > 0), () => (p.TenantId == value(My.Project.MyContext).CurrentTenantId), () => p)' specified for entity type 'MyEntity' is invalid. The expression must accept a single parameter of type 'My.Project.MyEntity' and return bool.'

我的问题是 - 我怎样才能实现我的目标？

Answer 1

类似的东西。您错过了将 parameter 添加到 Lambda 并在使用 lambda 时出错。还选择了最短的方法来做到这一点。

var parameter = Expression.Parameter(entityType.ClrType, "p");

// EF just needs to know that you have member access to DbContext
var contextExpr = Expression.Constant(null, typeof(MyContext));
var tenantExpr = Expression.MakeMemberAccess(contextExpr, currentTenantIdMemberInfo);

var conditionalExpression = Expression.OrElse(
    // Check if TenantId <= 0
    Expression.LessThanOrEqual(
        tenantExpr,
        Expression.Constant((long) 0)
        ),
        Expression.Equal(
            Expression.Property(parameter, colName),
            tenantExpr
        )
    )
);

var filter = Expression.Lambda(conditionalExpression, parameter);

entityType.SetQueryFilter(filter);

另外你并不孤单，这个任务已经被很多图书馆完成了，例如ASP.NET Boilerplate

为了简化此类任务，我强烈建议使用表达式树可视化器： https://github.com/agileobjects/ReadableExpressions https://github.com/zspitz/ExpressionTreeVisualizer