如何告诉 LINQ 如果条件为 false 则跳过 where？

Question

参考下面的代码：

而不是有这么多 if else 语句：

List<T> GetTs(string createBy, bool isAdmin, string departmentCode, ...)
{
    if(isAdmin)
    {
        if(!string.IsNullOrEmpty(createBy))
        {
            var query = context.table
                        .Where(x => string.compare(x.createBy, createBy, StringComparison.OrdinalIgnoreCase) == 0)
                        .ToList();
            return query;
        }
        else
        {
            var query = context.table.ToList();
            return query;
        }
    }
    else
    {
        if(!string.IsNullOrEmpty(createBy))
        {
            var query = context.table
                        .Where(x => string.compare(x.departmentCode, departmentCode, StringComparison.OrdinalIgnoreCase) == 0)
                        .Where(x => string.compare(x.createBy, createBy, StringComparison.OrdinalIgnoreCase) == 0)
                        .ToList();
            return query;
        }
        else
        {
            var query = context.table
                        .Where(x => string.compare(x.departmentCode, departmentCode, StringComparison.OrdinalIgnoreCase) == 0)
                        .ToList();
            return query;
        }
    }
}

我可以在没有那么多 if else 语句的情况下这样做吗：

var query  = context.table
                .Where(x => (!isAdmin)? string.compare(x.departmentCode, departmentCode, StringComparison.OrdinalIgnoreCase) == 0) : SKIP THIS '.Where')
                .Where(x => (!string.IsNullOrEmpty(createBy))? string.compare(x.createBy, createBy, StringComparison.OrdinalIgnoreCase) == 0) : SKIP THIS '.Where')
                //...
                .ToList();

如何告诉 LINQ 跳过.Whereif 条件为 false？可能的？

以下评论中帕特里克·霍夫曼的建议有效：

var query  = context.table
                .Where(x => (!isAdmin)? string.compare(x.departmentCode, departmentCode, StringComparison.OrdinalIgnoreCase) == 0) : true)
                .Where(x => (!string.IsNullOrEmpty(createBy))? string.compare(x.createBy, createBy, StringComparison.OrdinalIgnoreCase) == 0) : true)
                //...
                .ToList();

Answer 1

如果我理解意图（有条件地应用谓词），那么基本上：以不同的方式进行组合：

IQueryable<Whatever> query = context.Table;
if(condition1)
    query = query.Where(x => x.column1);
if(condition2)
    query = query.Where(x => x.column2);
//...etc
var list = query.ToList();

您可能可以将其包装在扩展方法中，例如

static IQueryable<T> WhereIf<T>(this IQueryable<T> query, bool condition,
        Expression<Func<T,bool>> predicate)
    => condition ? query.Where(predicate) : query;

和：

var list= context.Table.WhereIf(condition1, x => x.column1)
                       .WhereIf(condition2, x => x.column2)
                       // ...
                       .ToList();

但是，我可能不会这样做，因为在许多情况下，它需要构建不必要的附加表达式树。