如何声明Linq表达式变量以使其作为dbParameter处理

Question

我正在尝试针对Entity框架(或其他Linq提供程序)创建动态查询.

让我用一些示例代码解释我的问题.

如果我硬编码表达式:

var id = 12345;

Expression<Func<ItemSearch, bool>> myLambda = (s) => s.Id == id;

var finalQuery = context.ItemSearch.Where(myLambda);
this.Log((finalQuery as ObjectQuery).ToTraceString());

生成的SQL如下所示:

SELECT ...
FROM ViewItemSearch "Extent1"
WHERE "Extent1".ID = :p__linq__0

用一个很好的:p__linq__0dbParameter.

如果我创建一个表达式:

var id = 12345;
ParameterExpression param = Expression.Parameter(typeof(ItemSearch), "s");
Expression prop = Expression.Property(param, "Id");
Expression val = Expression.Constant(id);
Expression searchExpr = Expression.Equal(prop, val);

Expression<Func<ItemSearch, bool>> myLambda =  
    Expression.Lambda<Func<ItemSearch, bool>>(searchExpr , param);

var finalQuery = context.ItemSearch.Where(myLambda);
this.Log((finalQuery as ObjectQuery).ToTraceString());

生成的SQL如下所示:

SELECT ...
FROM ViewItemSearch "Extent1"
WHERE "Extent1".ID = 12345

不再有:p__linq__0dbParameter,因此Db引擎无法缓存查询计划.

我明白这是因为我使用

Expression val = Expression.Constant(id);

但我无法弄清楚如何绑定变量而不是值.

Answer 1

你需要在编译时创建一个lambda来关闭id变量,然后你可以在更复杂的lambda的组合中获取一个用法的主体:

var id = 12345;
ParameterExpression param = Expression.Parameter(typeof(ItemSearch), "s");
Expression prop = Expression.Property(param, "Id");
Expression<Func<int>> idLambda = () => id;
Expression searchExpr = Expression.Equal(prop, idLambda.Body);

Expression<Func<ItemSearch, bool>> myLambda =
    Expression.Lambda<Func<ItemSearch, bool>>(searchExpr, param);