---
id: "M:cAlgo.API.BacktestingDataRequest.Fail(System.String)"
kind: "method"
type: "T:cAlgo.API.BacktestingDataRequest"
url: "https://help.ctrader.com/ctrader-algo/references/Plugin/Backtesting/DataSource/BacktestingDataRequest/Fail/"
---

> Documentation index for agents: https://help.ctrader.com/ctrader-algo/llms.txt · machine index: https://help.ctrader.com/ctrader-algo/api.json

# BacktestingDataRequest.Fail

Member of [BacktestingDataRequest](https://help.ctrader.com/ctrader-algo/references/Plugin/Backtesting/DataSource/BacktestingDataRequest/index.md).

## Fail(string errorMessage)

Notifies current request failure if something went wrong.

Environments: Desktop supported · CLI not supported · Cloud not supported.

- CLI: Available to plugins only, and plugins cannot run in the CLI.
- Cloud: Available to plugins only, and plugins cannot run in the cloud.

C#:

```csharp
void Fail(string errorMessage)
```

Python:

```python
backtestingDataRequest.Fail(errorMessage: str) -> None
```

### Parameters

- `errorMessage` (`string`, Python `str`): Error message.

Available since cAlgo.API 5.3.

### Examples

C#:

```csharp
private void OnDataRequested(BacktestingDataRequest request)
{
    if (request.PriceDataSourceId.Symbol.Name != "EURUSD")
    {
        request.Fail($"Symbol {request.PriceDataSourceId.Symbol.Name} is not supported");
        return;
    }
    request.Complete(new BacktestingBarsData(Array.Empty<BacktestingBar>()));
}
```

Python:

```python
from System import Array

def on_data_requested(self, request):
    if request.PriceDataSourceId.Symbol.Name != "EURUSD":
        request.Fail(f"Symbol {request.PriceDataSourceId.Symbol.Name} is not supported")
        return
    request.Complete(BacktestingBarsData(Array[BacktestingBar]([])))
```