注意：
x => x.DTBUHOTCHET == '01.01.2016'）。适用于func：
Func<ScrollLineDTO, bool> predicate = x => x.DTBUHOTCHET == '01.01.2016';
Func<krt_Naftan, bool> func = x => predicate(Mapper.Map<ScrollLineDTO>(x));
因为我无法将func包装到Expression树（LINQ to SQL） //不适用于EF6
Expression<Func<krt_Naftan, bool>> filter = x => func(x);
我尝试在表达式中转换类型（编译错误）
Expression<Func<ScrollLineDTO, bool>> predicate = x => x.DTBUHOTCHET == '01.01.2016';
Expression<Func<krt_Naftan, bool>> func = x =>predicate(Mapper.Map<ScrollLineDTO>(x));
问题：如何在表达式树语句中使用转换功能？或者可能需要别的东西;）
1）索引方法（UI层）
public ActionResult Index(DateTime? period = null, int page = 1, bool asService = false, ushort initialSizeItem = 15) {
if (Request.IsAjaxRequest()) {
long recordCount;
//default
Expression<Func<ScrollLineDTO, bool>> predicate = x => x.DTBUHOTCHET == (period == null ? new DateTime(DateTime.Today.Year, DateTime.Today.Month, 1) : period);
var result = new IndexMV() {
ListKrtNaftan = _bussinesEngage.SkipTable(page, initialSizeItem, out recordCount, predicate),
...
}
};
....
2）（BLL层）
public IEnumerable<T> SkipTable<T>(int page, int initialSizeItem, out long recordCount, Expression<Func<T, bool>> predicate = null) {
if (predicate == null) {
//convert func types
Expression<Func<krt_Naftan, bool>> func = x => predicate(Mapper.Map<ScrollLineDTO>(x));
//wrap in func to expression (is not impossible, maybe if pass method...)
//Expression<Func<krt_Naftan, bool>> filter = x => func(x);
return (IEnumerable<T>)Mapper.Map<IEnumerable<ScrollLineDTO>>(Engage.GetSkipRows(page, initialSizeItem, out recordCount, x => x.KEYKRT, func));
}
return (IEnumerable<T>)Mapper.Map<IEnumerable<ScrollLineDTO>>(Engage.GetSkipRows<krt_Naftan, long>(page, initialSizeItem, out recordCount, x => x.KEYKRT));
}
/// <summary>
/// Return pagging part of table and general count of rows
/// </summary>
/// <typeparam name="T">Current enity</typeparam>
/// <typeparam name="TKey">Type for ordering</typeparam>
/// <param name="page">Number page</param>
/// <param name="size">Count row per one page</param>
/// <param name="recordCount"></param>
/// <param name="orderPredicate">Condition for ordering</param>
/// <param name="filterPredicate">Condition for filtering</param>
/// <param name="caсhe"></param>
/// <returns>Return definition count rows of specific entity</returns>
public IEnumerable<T> GetSkipRows<T, TKey>(int page, int size, out long recordCount, Expression<Func<T, TKey>> orderPredicate, Expression<Func<T, bool>> filterPredicate = null, bool caсhe = false) where T : class {
recordCount = GetCountRows(filterPredicate);
using (Uow = new UnitOfWork()) {
return Uow.Repository<T>().Get_all(filterPredicate, caсhe).OrderByDescending(orderPredicate).Skip((page - 1) * size).Take(size).ToList();
}
}
3）（DLL层）从db
获取数据/// <summary>
/// Get lazy data set (with cashing or not (attr MergeOption )
/// </summary>
/// <param name="predicate">filter condition for retrieving data from source(database)</param>
/// <param name="enableDetectChanges">Compare two snapshot of data (one when retrieve data from database other when call method saveChanges(). If exists some diffrences => generate avaible SQL command</param>
/// <param name="enableTracking"></param>
/// <returns></returns>
public IQueryable<T> Get_all(Expression<Func<T, bool>> predicate = null, bool enableDetectChanges = true, bool enableTracking = true) {
/*//sync data in Db & EF (if change not tracking for EF)
((IObjectContextAdapter)_context).ObjectContext.Refresh(RefreshMode.StoreWins, _dbSet.Where(predicate));
_context.Entry(_dbSet.Where(predicate)).Reload(); EF 4.1+*/
ActiveContext.Configuration.AutoDetectChangesEnabled = enableDetectChanges;
if (predicate == null) return (enableTracking) ? _dbSet : _dbSet.AsNoTracking();
var result = (enableTracking) ? _dbSet.Where(predicate) : _dbSet.AsNoTracking().Where(predicate);
return result;
}
THX）
答案 0 :(得分：1)
我找到了我需要的东西！
Solusion是访问者模式（lINQ具有内置ExpressionVisitor实现（С＃4.0））。
在我的案例中，实现是：
public static Expression<Func<OutT, bool>> ConvertTypeExpression<inT, OutT>(Expression expression) where OutT : class {
var param = Expression.Parameter(typeof(OutT), "x");
var result = new CustomExpVisitor<OutT>(param).Visit(expression);
Expression<Func<OutT, bool>> lambda = Expression.Lambda<Func<OutT, bool>>(result, new[] { param });
return lambda;
}
private class CustomExpVisitor<T> : ExpressionVisitor {
ParameterExpression _param;
public CustomExpVisitor(ParameterExpression param) {
_param = param;
}
protected override Expression VisitParameter(ParameterExpression node) {
return _param;
}
protected override Expression VisitMember(MemberExpression node) {
if (node.Member.MemberType == MemberTypes.Property) {
MemberExpression memberExpression = null;
var memberName = node.Member.Name;
var otherMember = typeof(T).GetProperty(memberName);
memberExpression = Expression.Property(Visit(node.Expression), otherMember);
return memberExpression;
} else {
return base.VisitMember(node);
}
}
}
最后我得到了我搜索过的函数
var filterPredicate = PredicateExtensions.ConvertTypeExpression<ScrollLineDTO, krt_Naftan>(predicate.Body);
答案 1 :(得分：0)
此解决方案适用于单个表达式（x =＆gt; x.TypeId ==“MyType”），但此时它在生产中有效。
T1和T2不需要彼此分配。
步骤如下： 1.将表达式转换为字典 2.生成等式表达式（您可以实现其他比较）
我随着时间的推移将这些库拼凑在一起，不记得它们的来源，但这里是主要的方法。
public class ExpressionHelper
{
public static IDictionary<string, object> GetMethodParams<T>(Expression<Func<T, bool>> fromExpression)
{
if (fromExpression == null) return null;
var body = fromExpression.Body as BinaryExpression;
if (body == null) return new Dictionary<string, object>();
var rVal = new Dictionary<string, object>();
var leftLambda = body.Left as BinaryExpression;
if (leftLambda != null)
{
var params1 = GetExpressionParams(leftLambda);
foreach (var o in params1) rVal.Add(o.Key, o.Value);
}
var rightLambda = body.Right as BinaryExpression;
if (rightLambda != null)
{
var params1 = GetExpressionParams(rightLambda);
foreach (var o in params1) rVal.Add(o.Key, o.Value);
}
else
{
var params1 = GetExpressionParams(body);
foreach (var o in params1) rVal.Add(o.Key, o.Value);
}
return rVal;
}
/// <summary>
/// Get Expression Parameters Recursively
/// </summary>
/// <param name="body"></param>
/// <returns></returns>
private static IDictionary<string, object> GetExpressionParams(BinaryExpression body)
{
if (body == null) return new Dictionary<string, object>();
var rVal = new Dictionary<string, object>();
var leftLambda = body.Left as BinaryExpression;
while (leftLambda != null)
{
var params1 = GetExpressionParams(leftLambda);
foreach (var o in params1) if (!rVal.ContainsKey(o.Key)) rVal.Add(o.Key, o.Value);
leftLambda = body.Left as BinaryExpression;
}
var rightLambda = body.Right as BinaryExpression;
while (rightLambda != null)
{
var params1 = GetExpressionParams(rightLambda);
foreach (var o in params1) if (!rVal.ContainsKey(o.Key)) rVal.Add(o.Key, o.Value);
rightLambda = body.Right as BinaryExpression;
}
var rightValue = GetValue(body.Right);
if (rightValue == null)
{
var rightSide = body.Right as ConstantExpression;
if (rightSide != null) rightValue = rightSide.Value;
}
var leftSideName = GetMemberName(body.Left);
if (string.IsNullOrEmpty(leftSideName)) return rVal;
if (!rVal.ContainsKey(leftSideName)) rVal.Add(leftSideName, rightValue);
return rVal;
}
/// <summary>
/// Get an Equals Expression from the name and value
/// </summary>
/// <typeparam name="T"></typeparam>
/// <param name="propertyName"></param>
/// <param name="value"></param>
/// <returns></returns>
public static Expression<Func<T, bool>> GetEqualExpression<T>(string propertyName, object value)
{
var p = Expression.Parameter(typeof(T));
var property = Expression.Property(p, propertyName);
Expression propertyExpression = Expression.Call(property, property.Type.GetMethod("ToString", Type.EmptyTypes));
var equalsExpression = Expression.Equal(propertyExpression, Expression.Constant(value?.ToString()));
var lambda = Expression.Lambda<Func<T, bool>>(equalsExpression, p);
return lambda;
}
}
使用具有相同属性StatusId的项目。
public class DataReader
{
/// <summary>
/// Get your list via data manager or something
/// </summary>
public List<T> ListItems<T>()
where T : IStatusIdProperty
{
return new List<T>();
}
public List<T> GetPubItems<T, TView>()
where T:IStatusIdProperty
where TView : IStatusIdProperty
{
var expression = ConvertExpression<T, TView>(x => x.StatusId == "Pub");
return ListItems<T>().Where(expression.Compile()).ToList();
}
public Expression<Func<T, bool>> ConvertExpression<T, TView>(Expression<Func<TView, bool>> predicate)
where T : IStatusIdProperty
where TView : IStatusIdProperty
{
var paramDictionary = predicate.GetParamsDictionary().FirstOrDefault();
var expression = ExpressionHelper.GetEqualExpression<T>(paramDictionary.Key, paramDictionary.Value);
return expression;
}
}