我有以下基于列名和值构建表达式的方法:
public Func<TSource, bool> SimpleComparison<TSource>(string property, object value)
{
var type = typeof (TSource);
var pe = Expression.Parameter(type, "p");
var propertyReference = Expression.Property(pe, property);
var constantReference = Expression.Constant(value);
return Expression.Lambda<Func<TSource, bool>>
(Expression.Equal(propertyReference, constantReference),
new[] { pe }).Compile();
}
我正在尝试创建一个表达式列表,然后使用最终的级联/编译表达式查询我的数据源。
我曾尝试使用Expression.Block
,但这个概念对我来说很难理解。我也看过Expression.Loop
,但不确定是否要在这里使用。
理想情况下,我可以执行以下操作:
var filters = request.Filter.Filters;
IQueryable<MyDTO> myDataSource = context.MyDataSource;
var expressions = null;
foreach (var filter in filters)
{
expressions.Add(SimpleExpression<MyDTO>(filter.Name, filter.Value));
}
return myDataSource.Where(expressions);
关于如何执行类似操作的任何想法?
SimpleComparison
,但您已经完成了。好吧,主要是。您应该返回Expression<Func<...>>
,而不是Func<...>
,因此应该是public Expression<Func<TSource, bool>> SimpleComparison<TSource>(string property, object value)
{
// ...
return Expression.Lambda<Func<TSource, bool>>
(Expression.Equal(propertyReference, constantReference),
new[] { pe });
}
一旦有了,就可以通过重复调用Where
来链接过滤器,如下所示:
var filters = request.Filter.Filters;
IQueryable<MyDTO> query = context.MyDataSource;
foreach (var filter in filters)
query = query.Where(SimpleComparison<MyDTO>(filter.Name, filter.Value));
return query;
Aggregate()
函数来构建最终谓词。public Func<TSource, bool> CreatePredicate<TSource>(IEnumerable<Expression> expressions)
{
var parameter = Expression.Parameter(typeof(TSource), "x");
var body = expressions.Aggregate((e, next) => Expression.AndAlso(e, next));
var predicate = Expression.Lambda<Func<TSource, bool>>(body, parameter).Compile();
return predicate;
}
这是假设您希望使用&&
将所有语句连接在一起。如果要使用||
将它们加入,请使用Expression.OrElse
代替Expression.AndAlso
。如果混合使用和和或和过滤器,则解决方案将变得更加复杂。您上面的示例将成为
var filters = request.Filter.Filters;
IQueryable<MyDTO> myDataSource = context.MyDataSource;
var expressions = new List<Expression>();
foreach (var filter in filters)
{
expressions.Add(SimpleComparison<MyDTO>(filter.Name, filter.Value));
}
var predicate = CreatePredicate<MyDTO>(expressions);
return myDataSource.Where(predicate);
public Expression<Func<TSource, bool>> SimpleComparison<TSource>(List<QueryFilterObject> queryFilterObjects)
{
//initialize the body expression
BinaryExpression bodyExpression = null;
BinaryExpression andExpressionBody = null;
BinaryExpression orExpressionBody = null;
//create parameter expression
ParameterExpression parameterExpression = Expression.Parameter(typeof(TSource), "DynamicFilterQuery");
//list of binary expressions to store either the || or && operators
List<BinaryExpression> andExpressions = new List<BinaryExpression>();
List<BinaryExpression> orExpressions = new List<BinaryExpression>();
foreach (var queryFilterObject in queryFilterObjects)
{
//create member property expression
var property = Expression.Property(parameterExpression, queryFilterObject.PropertyName);
//create the constant expression value
var constantExpressionValue = Expression.Constant(queryFilterObject.Value, queryFilterObject.PropertyType);
//create the binary expression clause based on the comparison operator
BinaryExpression clause = null;
if (queryFilterObject.ComparisonOperator == "==")
{
clause = Expression.Equal(property, constantExpressionValue);
}
else if (queryFilterObject.ComparisonOperator == "!=")
{
clause = Expression.NotEqual(property, constantExpressionValue);
}
else if (queryFilterObject.ComparisonOperator == ">")
{
clause = Expression.GreaterThan(property, constantExpressionValue);
}
else if (queryFilterObject.ComparisonOperator == ">=")
{
clause = Expression.GreaterThan(property, constantExpressionValue);
}
else if (queryFilterObject.ComparisonOperator == "<")
{
clause = Expression.LessThan(property, constantExpressionValue);
}
else if (queryFilterObject.ComparisonOperator == "<=")
{
clause = Expression.LessThanOrEqual(property, constantExpressionValue);
}
//you should validate against a null clause....
//assign the item either to the relevant logical comparison expression list
if (queryFilterObject.LogicalOperator == "and" || queryFilterObject.LogicalOperator == "&&")
{
andExpressions.Add(clause);
}
else if (queryFilterObject.LogicalOperator == "or" || queryFilterObject.LogicalOperator == "||")
{
orExpressions.Add(clause);
}
}
if (andExpressions.Count > 0)
andExpressionBody = andExpressions.Aggregate((e, next) => Expression.And(e, next));
if (orExpressions.Count > 0)
orExpressionBody = orExpressions.Aggregate((e, next) => Expression.Or(e, next));
if (andExpressionBody != null && orExpressionBody != null)
bodyExpression = Expression.OrElse(andExpressionBody, orExpressionBody);
if (andExpressionBody != null && orExpressionBody == null)
bodyExpression = andExpressionBody;
if (andExpressionBody == null && orExpressionBody != null)
bodyExpression = orExpressionBody;
if (bodyExpression == null)
throw new Exception("Null Expression.");
var finalExpression = Expression.Lambda<Func<WorkOrder, bool>>(bodyExpression, parameterExpression);
return finalExpression;
}
public class QueryFilterObject
{
public string PropertyName { get; set; }
public Type PropertyType { get; set; }
public object Value { get; set; }
public string ComparisonOperator { get; set; }
public string LogicalOperator { get; set; }
}
就我而言,我返回的是func(Expression<Func<TSource, bool>>
)表达式,而不是func本身(Func<TSource, bool>
)。这允许我的where子句保留为Iqueryable,否则返回[
最后,我就像下面这样称呼我的表情:Func<TSource, bool>
而不是*Expression<Func<TSource, bool>>*
会将您的where子句转换为ienumerable。
IQueryable<MyDTO> myDataSource = context.MyDataSource; var filter = SimpleComparison(queryFilterObjects); if (filter != null) myDataSource = myDataSource.Where(filter); //perfom other operations such as order by return myDataSource.ToList();