使用Expression进行查询拼接

当我们使用lambda表达式进行查询的时候，查询的条件能进行动态拼接么？

答案是可以

public static class DynamicLinqExpressions
{

public static Expression<Func<T, bool>> True<T>() { return f => true; }
public static Expression<Func<T, bool>> False<T>() { return f => false; }

public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> expr1,
Expression<Func<T, bool>> expr2)
{
var invokedExpr = Expression.Invoke(expr2, expr1.Parameters.Cast<Expression>());
return Expression.Lambda<Func<T, bool>>
(Expression.Or(expr1.Body, invokedExpr), expr1.Parameters);
}

public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> expr1,
Expression<Func<T, bool>> expr2)
{
var invokedExpr = Expression.Invoke(expr2, expr1.Parameters.Cast<Expression>());
return Expression.Lambda<Func<T, bool>>
(Expression.And(expr1.Body, invokedExpr), expr1.Parameters);
}

}

上面贴上的代码就提供了将两个lambda表达式拼接到一块的函数or和and。

应该怎么使用呢？

public static IEnumerable<T> FilterBy<T>(this IEnumerable<T> collection, IEnumerable<KeyValuePair<string, string>> query)
{
var filtersCount = int.Parse(query.SingleOrDefault(k => k.Key.Contains("filterscount")).Value);

Expression<Func<T, bool>> finalquery = null;

for (var i = 0; i < filtersCount; i += 1)
{
var filterValue = query.SingleOrDefault(k => k.Key.Contains("filtervalue" + i)).Value;
var filterCondition = query.SingleOrDefault(k => k.Key.Contains("filtercondition" + i)).Value;
var filterDataField = query.SingleOrDefault(k => k.Key.Contains("filterdatafield" + i)).Value;
var filterOperator = query.SingleOrDefault(k => k.Key.Contains("filteroperator" + i)).Value;

Expression<Func<T, bool>> current = n => GetQueryCondition(n, filterCondition, filterDataField, filterValue);

if (finalquery == null)
{
finalquery = current;
}
else if (filterOperator == "1")
{
finalquery = finalquery.Or(current);
}
else
{
finalquery = finalquery.And(current);
}
};

if (finalquery != null)
collection = collection.AsQueryable().Where(finalquery);

return collection;
}

注意看上述Or和And完成了两个条件的拼接，最终在where里面执行查询。