我创建了一个通用表达式构建器，它根据条件集合构建谓词。我将谓词传递给存储库中的泛型方法。我认为表达式构建器工作正常并创建所需的谓词，尽管实体框架生成的SQL脚本不是我预期的。我已经阅读了很多有关此问题的动态查询或LinqKit和表达式构建器的问题和文章，最相关的是this comment。如果你能看看我做了什么，我真的很感激，如果我犯了任何错误，请告诉我？
以下是ExpressionBuilder类的代码：
public static class ExpressionBuilder
{
private static MethodInfo containsMethod = typeof(string).GetMethod("Contains");
private static MethodInfo startsWithMethod = typeof(string).GetMethod("StartsWith", new Type[] { typeof(string) });
private static MethodInfo endsWithMethod = typeof(string).GetMethod("EndsWith", new Type[] { typeof(string) });
public static Expression<Func<T, bool>> GetExpression<T>(IList<ExpressionModel> filters)
{
if (filters == null)
return null;
IList<ExpressionModel> nullFreeCollection = filters.OfType<ExpressionModel>().ToList();
if (nullFreeCollection.Count == 0)
return null;
ParameterExpression param = Expression.Parameter(typeof(T), "item");
Expression exp = null;
if (nullFreeCollection.Count == 1)
exp = GetExpression<T>(param, nullFreeCollection[0]);
else if (nullFreeCollection.Count == 2)
exp = GetExpression<T>(param, nullFreeCollection[0], nullFreeCollection[1]);
else
{
while (nullFreeCollection.Count > 0)
{
var f1 = nullFreeCollection[0];
var f2 = nullFreeCollection[1];
if (exp == null)
exp = GetExpression<T>(param, nullFreeCollection[0], nullFreeCollection[1]);
else
exp = Expression.AndAlso(exp, GetExpression<T>(param, nullFreeCollection[0], nullFreeCollection[1]));
nullFreeCollection.Remove(f1);
nullFreeCollection.Remove(f2);
if (nullFreeCollection.Count == 1)
{
exp = Expression.AndAlso(exp, GetExpression<T>(param, nullFreeCollection[0]));
nullFreeCollection.RemoveAt(0);
}
}
}
return Expression.Lambda<Func<T, bool>>(exp, param);
}
private static Expression GetExpression<T>(ParameterExpression param, ExpressionModel filter)
{
MemberExpression member = Expression.Property(param, filter.PropertyName);
ConstantExpression constant = Expression.Constant(filter.Value);
switch (filter.Operator)
{
case ExpressionOperators.Equals:
return Expression.Equal(member, constant);
case ExpressionOperators.GreaterThan:
return Expression.GreaterThan(member, constant);
case ExpressionOperators.LessThan:
return Expression.LessThan(member, constant);
case ExpressionOperators.GreaterThanOrEqual:
return Expression.GreaterThanOrEqual(member, constant);
case ExpressionOperators.LessThanOrEqual:
return Expression.LessThanOrEqual(member, constant);
case ExpressionOperators.Contains:
return Expression.Call(member, containsMethod, constant);
case ExpressionOperators.StartsWith:
return Expression.Call(member, startsWithMethod, constant);
case ExpressionOperators.EndsWith:
return Expression.Call(member, endsWithMethod, constant);
}
return null;
}
private static BinaryExpression GetExpression<T>(ParameterExpression param, ExpressionModel filter1, ExpressionModel filter2)
{
Expression bin1 = GetExpression<T>(param, filter1);
Expression bin2 = GetExpression<T>(param, filter2);
return Expression.AndAlso(bin1, bin2);
}
public enum ExpressionOperators
{
Equals,
GreaterThan,
LessThan,
GreaterThanOrEqual,
LessThanOrEqual,
Contains,
StartsWith,
EndsWith
}
}
以下是通用存储库方法：
public IEnumerable<TEntity> RetrieveCollectionAsync(Expression<Func<TEntity, bool>> predicate)
{
try
{
return DataContext.Set<TEntity>().Where(predicate);
}
catch (Exception ex)
{
Logger.Error(ex);
throw ex;
}
}
由Entity Framework for Sql生成的脚本（我期望带有一些where子句的select查询）：
SELECT
CAST(NULL AS uniqueidentifier) AS [C1],
CAST(NULL AS uniqueidentifier) AS [C2],
CAST(NULL AS varchar(1)) AS [C3],
CAST(NULL AS uniqueidentifier) AS [C4],
CAST(NULL AS uniqueidentifier) AS [C5],
CAST(NULL AS uniqueidentifier) AS [C6],
CAST(NULL AS datetime2) AS [C7],
CAST(NULL AS datetime2) AS [C8],
CAST(NULL AS varchar(1)) AS [C9],
CAST(NULL AS uniqueidentifier) AS [C10],
CAST(NULL AS varchar(1)) AS [C11],
CAST(NULL AS uniqueidentifier) AS [C12],
CAST(NULL AS uniqueidentifier) AS [C13],
CAST(NULL AS uniqueidentifier) AS [C14],
CAST(NULL AS uniqueidentifier) AS [C15],
CAST(NULL AS datetime2) AS [C16],
CAST(NULL AS varchar(1)) AS [C17],
CAST(NULL AS datetime2) AS [C18],
CAST(NULL AS varchar(1)) AS [C19],
CAST(NULL AS tinyint) AS [C20]
FROM ( SELECT 1 AS X ) AS [SingleRowTable1]
WHERE 1 = 0
我正在使用
更新表达式的模型：
public class ExpressionModel
{
public string PropertyName { get; set; }
public ExpressionOperators Operator { get; set; }
public object Value { get; set; }
}
另一个缺失的部分是通用映射器，它将给定的搜索条件映射到新的ExpressionModel，我认为它与此问题无关。
答案 0 :(得分：4)
正如我在评论中提到的，实施过于复杂。
首先，这个方法
private static BinaryExpression GetExpression<T>(ParameterExpression param, ExpressionModel filter1, ExpressionModel filter2)
并且用于检查过滤器计数，删除已处理项目等的整个逻辑是多余的。 AND条件可以像这样轻松链接
((Condition1 AND Condition2) AND Condition3) AND Condition4 ...
所以只需删除该功能。
其次，这个功能
private static Expression GetExpression<T>(ParameterExpression param, ExpressionModel filter)
命名不佳，不需要通用T，因为它没有在内部使用。
而是将签名更改为
private static Expression MakePredicate(ParameterExpression item, ExpressionModel filter)
{
// implementation (same as posted)
}
最后，公共方法很简单：
public static Expression<Func<T, bool>> MakePredicate<T>(IEnumerable<ExpressionModel> filters)
{
if (filters == null) return null;
filters = filters.Where(filter => filter != null);
if (!filters.Any()) return null;
var item = Expression.Parameter(typeof(T), "item");
var body = filters.Select(filter => MakePredicate(item, filter)).Aggregate(Expression.AndAlso);
var predicate = Expression.Lambda<Func<T, bool>>(body, item);
return predicate;
}
P.S。并且不要忘记null检查用法：
// should not be called Async
public IEnumerable<TEntity> RetrieveCollectionAsync(Expression<Func<TEntity, bool>> predicate)
{
try
{
var query = DataContext.Set<TEntity>().AsQueryable();
if (predicate != null)
query = query.Where(predicate);
return query;
}
catch (Exception ex)
{
Logger.Error(ex);
throw ex; // should be: throw;
}
}