Blazor 选择绑定到列表中的值 [重复]

Question

我能够将 int 类型（这是 SelectedDiagnosisIdList 列表的第一个元素）变量绑定到 html 选择的选定元素：

<div>
    <label>Diagnosis:</label>
    <div>
        <select @bind="@userInfo.SelectedDiagnosisIdList[0]">
            @foreach (var item in diagnoses)
            {
                <option value="@item.Id">@item.Name</option>
            }
        </select>
        <p>@userInfo.SelectedDiagnosisIdList[0]"</p>
    </div>
</div>

@code
{
    List<int> SelectedDiagnosisIdList = new List<int>() { 0 };
    List<Diagnosis> diagnoses; //  populated from db in OnInitializedAsync

}

这很好用，当我更改 UI 上的选定值时，段落的值会发生变化。

---

现在，我想添加更多的Diagnoses，所以我试图维护更多的s，并在SelectedDiagnosisIdList中添加更多的元素：

<div>
    <label>Diagnosis:</label>
    <div>
        @for(int i = 0; i < userInfo.SelectedDiagnosisIdList.Count; i++)
            <select @bind="@userInfo.SelectedDiagnosisIdList[i]">
                @foreach (var item in diagnoses)
                {
                    <option value="@item.Id">@item.Name</option>
                }
            </select>
            <p>@userInfo.SelectedDiagnosisIdList[i]</p>
        }
    </div>
</div>

@code
{
    List<int> SelectedDiagnosisIdList = new List<int>() { 0 };
    List<Diagnosis> diagnoses; //  populated from db in OnInitializedAsync

}

这会破坏程序，当我选择一个项目和控制台时，UI 上没有任何反应：

blazor.webassembly.js:1 WASM: Unhandled exception rendering component:
blazor.webassembly.js:1 WASM: System.ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection.
WASM: Parameter name: index
blazor.webassembly.js:1 WASM:   at (wrapper managed-to-native) System.Reflection.RuntimeMethodInfo.InternalInvoke(System.Reflection.RuntimeMethodInfo,object,object[],System.Exception&)
blazor.webassembly.js:1 WASM:   at System.Reflection.RuntimeMethodInfo.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) <0x20c1360 + 0x000ce> in <71c4fd446b1842ba93fd332e5f4f9c06>:0 
blazor.webassembly.js:1 WASM: --- End of stack trace from previous location where exception was thrown ---
blazor.webassembly.js:1 WASM:   at Microsoft.AspNetCore.Components.ComponentBase.CallStateHasChangedOnAsyncCompletion (System.Threading.Tasks.Task task) <0x269de40 + 0x000e6> in <cbf249d7f04d4fa18d15bfae8ef7f389>:0 
WASM:   at Microsoft.AspNetCore.Components.RenderTree.Renderer.GetErrorHandledTask (System.Threading.Tasks.Task taskToHandle) <0x269e988 + 0x000c2> in <cbf249d7f04d4fa18d15bfae8ef7f389>:0 

我不明白问题出在哪里，我怎样才能让它工作......请帮助

Answer 1

我猜您的问题与 for 循环有关。您应该在循环中定义一个局部变量，如下所示：

    <div>
    @for(int i = 0; i < userInfo.SelectedDiagnosisIdList.Count; i++)
      {
        int local = i;
        <select id=$"dropdown{local+1}" 
               @bind="@userInfo.SelectedDiagnosisIdList[local]">
            @foreach (var item in diagnoses)
            {
                <option value="@item.Id">@item.Name</option>
            }
        </select>
        <p>@userInfo.SelectedDiagnosisIdList[local]</p>
    }
    </div>

希望这会有所帮助...