在Angular应用程序的编译中找不到声明的InjectionToken

我试图通过使用 InjectionToken 。但是,在编译应用程序的源代码中找不到声明的常量。我以前见过这种模式有效(1),但我不确定我做的有什么不同。下面是我的代码摘要。

// ./services/system/masks.service

export declare const NUMBER_MASK: InjectionToken<string>;

export function NumberMaskFactory(): NumberMask {
    const lang = navigator.language.substring(0, 2);
    if (lang == "en") {
        const radix: string = getLocaleNumberSymbol(navigator.language, NumberSymbol.Decimal);
        const thousandSeperator: string = getLocaleNumberSymbol(navigator.language, NumberSymbol.Group);

        return new NumberMask(radix, thousandSeperator);
    }
    return new NumberMask(".", ",");
}


export class NumberMask {
    public radix: string;
    public thousandSeparator: string;
    constructor(radix: string, thousandSeparator: string) {
        this.radix = radix;
        this.thousandSeparator = thousandSeparator;
    }
}

// app.module.ts
import { NumberMaskFactory, NUMBER_MASK } from "./services/system/masks.service";
// ...
providers: [
        {
            provide: NUMBER_MASK,
            useFactory: NumberMaskFactory

        }
// ...

编译时,这会引发以下错误:

./src/app/app。单元ts:112:21-32-错误:在中找不到导出“数字掩码”(导入为“数字掩码”)/服务/系统/口罩。服务(可能的出口:NumberMask、NumberMaskFactory)

(注1)例如: MSAL_INSTANCE 在这个 sample .