Question

我试图在用户输入字符串的地方进行搜索，然后在我的数据库中找到匹配的字符串。

目前，使用.Contains()进行搜索工作正常。例如。

User Input: Park 

Results: Central Park, Washing Square Park, Riverside Park, Park Avenue, Park Chester

但是，如果我使用.StartsWith()而不是.Contains()，则会导致抛出空引用异常。

var dbCode = await repo.GetAsync<List<DBCode>>(_ => _.Code.Equals(id) && _.Name.ToUpper().StartsWith(searchWord.ToUpper())); //Doesn't work, throws NRE 
var dbCode = await repo.GetAsync<List<DBCode>>(_ => _.Code.Equals(id) && _.Name.ToUpper().Contains(searchWord.ToUpper())); //works fine

我想使用.StartsWith()，所以我的预期结果如下：

User Input: Park 

Result: Park Avenue, Park Chester

而不是所有在公园结束的单词。

我如何使用上面使用的方法做到这一点？

以下例外情况：

System.NullReferenceException: Object reference not set to an instance of an object.
at SQLite.TableQuery`1.CompileExpr(Expression expr, List`1 queryArgs)
   at SQLite.TableQuery`1.CompileExpr(Expression expr, List`1 queryArgs)
   at SQLite.TableQuery`1.GenerateCommand(String selectionList)
   at SQLite.TableQuery`1.GetEnumerator()
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at SQLite.AsyncTableQuery`1.<ToListAsync>b__9_0()
   at System.Threading.Tasks.Task`1.InnerInvoke()
   at System.Threading.Tasks.Task.Execute()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at DSNY.Novas.Data.SQLiteRepository`1.<GetAsync>d__3`1.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()

此外，我在第一条评论中也使用了Generic Repository：Generic Repository for SQLite-Net in Xamarin Project

特别是：

 public async Task<List<T>> GetAsync<TValue>(Expression<Func<T, bool>> predicate = null,
            Expression<Func<T, TValue>> orderBy = null)
        {
            var query = Context.Table<T>();

            if (predicate != null)
                query = query.Where(predicate);

            if (orderBy != null)
                query = query.OrderBy<TValue>(orderBy);

            return await query.ToListAsync();
        }

使用Sqlite Async支持.StartsWith

0 个答案: