调用httpclient无法获取其内部的方法

代码如下：

import { _HttpClient } from '@delon/theme';
import { Observable } from 'rxjs';
import { map } from 'rxjs/operators';

// tslint:disable-next-line: class-name
export class tools {
   private http: _HttpClient;

  loadSelect(http, url, params): Observable<String[]> {
    const newUrl = url + params;
    const select = [];
    return this.http.get(newUrl).pipe(
      map((res: any) => {
        if (res.status === 0) {
          for (const x of res.data) {
            const obj = { label: '', value: '' };
            obj.label = x.title;
            obj.value = x.title;
            select.push(obj);
          }
        }
        return select;
      }),
    );
  }
}

报错如下：
ERROR TypeError: Cannot read property 'get' of undefined
at tools.loadSelect (tools.ts:12)

this.http为空，也就是_HttpClient为空，检查下'@delon/theme'，theme从字面上理解是主题样式的意思，是不是引用有误

return this.http.get(newUrl).pipe(
这里http没有get，http在哪里初始化的。

'@delon/theme'; 你看看get方法和你写的一直吗（参数方面）

你引用的库不对 一般是用 @angular/common/http
然后还需要添加构造方法 在里边注入http

import { HttpClient } from '@angular/common/http';
import { Observable } from 'rxjs';
import { map } from 'rxjs/operators';

// tslint:disable-next-line: class-name
export class tools {

  constructor(
    private http: HttpClient
  ) { }

  loadSelect(http, url, params): Observable<String[]> {
    const newUrl = url + params;
    const select = [];
    return this.http.get(newUrl).pipe(
      map((res: any) => {
        if (res.status === 0) {
          for (const x of res.data) {
            const obj = { label: '', value: '' };
            obj.label = x.title;
            obj.value = x.title;
            select.push(obj);
          }
        }
        return select;
      }),
    );
  }
}