如何解决如何为可空属性如 Nullable<Int32>
public static Expression<Func<TEntity,bool>> WhereFunc<TEntity>(this string propertyName,object? propertyValue)
where TEntity : class,IEntity
{
var info = typeof(TEntity).GetProperty(propertyName);
var parameter = Expression.Parameter(typeof(TEntity),$"{nameof(TEntity).Substring(0,2)}");
var property = Expression.Property(parameter,propertyName);
var value = Expression.Constant(propertyValue);
var clause = Expression.Equal(property,Expression.Constant(propertyValue));
return Expression.Lambda<Func<TEntity,bool>>(clause,parameter);
}
public static IQueryable<TEntity> SetWhere<TEntity,TSearch>(this IEnumerable<TEntity>
entities,IEnumerable<PropertyInfo> fields,TSearch entity)
where TEntity : class,IEntity
where TSearch : class,ISearchEntity
{
var query = entities.AsQueryable();
if (fields != null && fields.Any())
foreach (var item in fields)
query = query.Where(EntityFuncs.WhereFunc<TEntity>(item.Name,item.GetValue(entity)));
return query;
}
这是我使用它的代码:
var query = Entities
.Skip(filter.Total)
.Take(filter.More)
.AsQueryable();
query = query.SetWhere<TEntity,TSearchEntity>(properties,filter.Entity);
对于字符串属性,它可以正常工作。
但在 nullable<int>
属性的示例中,我收到此错误:
未为类型“System.Nullable`1[system.int32]”和“system.int32”定义二元运算符 Equal
解决方法
您必须显式测试 propertyValue
的类型或属性的类型,并相应地调用 Nullable.Equals
。
Nullable.GetUnderlyingType
构造的类型(例如 int
),Nullable<>
将返回不可为空的底层类型(例如 int?
);否则将返回 null
。
public static Expression<Func<TEntity,bool>> WhereFunc<TEntity>(this string propertyName,object? propertyValue)
where TEntity : class,IEntity
{
var info = typeof(TEntity).GetProperty(propertyName);
var parameter = Expression.Parameter(typeof(TEntity),$"{nameof(TEntity).Substring(0,2)}");
var property = Expression.Property(parameter,propertyName);
var value = Expression.Constant(propertyValue);
var propertyType = info?.PropertyType ?? propertyValue?.GetType();
var underlyingType = propertyType is not null ? Nullable.GetUnderlyingType(propertyType) : null;
Expression clause;
if (underlyingType is not null)
{
var methodOpen = typeof(Nullable).GetMethod(nameof(Nullable.Equals),BindingFlags.Static | BindingFlags.Public);
var method = methodOpen.MakeGenericMethod(underlyingType);
clause = Expression.Call(method,property,Expression.Convert(value,propertyType));
}
else
{
clause = Expression.Equal(property,value);
}
return Expression.Lambda<Func<TEntity,bool>>(clause,parameter);
}
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