角材料表投掷长度未定义错误，

“在此html文件中，这是说length属性是未定义的。当我使用   由角度cli生成的硬编码数据，当我使用该数据时，我没有收到error。   其余api响应，那时候我出错了。”

 <button type="button" class="btn btn-primary" (click)="OnFetchEmployee()">fetchEmpoyee Web service</button>
    <div class="mat-elevation-z8" data-table>
      <table mat-table class="full-width-table" matSort aria-label="Elements">
        <!-- Id Column -->
        <ng-container matColumnDef="id">
          <th mat-header-cell *matHeaderCellDef mat-sort-header>Id</th>
          <td mat-cell *matCellDef="let row">{{row.id}}</td>
        </ng-container>

        <!-- Name Column -->
        <ng-container matColumnDef="name">
          <th mat-header-cell *matHeaderCellDef mat-sort-header>Name</th>
          <td mat-cell *matCellDef="let row">{{row.name}}</td>
        </ng-container>

         <!-- emailId Column -->
         <ng-container matColumnDef="emailId">
          <th mat-header-cell *matHeaderCellDef mat-sort-header>Email</th>
          <td mat-cell *matCellDef="let row">{{row.emailId}}</td>
        </ng-container>

        <tr mat-header-row *matHeaderRowDef="displayedColumns"></tr>
        <tr mat-row *matRowDef="let row; columns: displayedColumns;"></tr>
      </table>

      <mat-paginator #paginator
          [length]="dataSource?.data.length"
          [pageIndex]="0"
          [pageSize]="50"
          [pageSizeOptions]="[1,25, 50, 100, 250]">
      </mat-paginator>
    </div>

“这是我在OnFetchEmployee（）中执行其余api调用的数据表组件”

import { AfterViewInit, Component, OnInit, ViewChild, Input } from '@angular/core';
import { MatPaginator } from '@angular/material/paginator';
import { MatSort } from '@angular/material/sort';
import { MatTable } from '@angular/material/table';
import { DataTableDataSource, DataTableItem } from './data-table-datasource';
import { Employee } from '../shared/employee.model';
import { RestCallService } from '../rest-call.service';

@Component({
  selector: 'app-data-table',
  templateUrl: './data-table.component.html',
  styleUrls: ['./data-table.component.css']
})
export class DataTableComponent implements AfterViewInit, OnInit {
  @ViewChild(MatPaginator, {static: false}) paginator: MatPaginator;
  @ViewChild(MatSort, {static: false}) sort: MatSort;
  @ViewChild(MatTable, {static: false}) table: MatTable<DataTableItem>;

  dataSource: DataTableDataSource;

  constructor(private restCall : RestCallService){}

  /** Columns displayed in the table. Columns IDs can be added, removed, or reordered. */
  displayedColumns = ['id', 'name' ,'emailId'];
  Employee :Employee[] =[];
  ngOnInit() {
    this.dataSource = new DataTableDataSource();
  }

  ngAfterViewInit() {
    this.dataSource.sort = this.sort;
    this.dataSource.paginator = this.paginator;
    this.table.dataSource = this.dataSource;
  }

  OnFetchEmployee(){

    console.log("onfetchEmployee is called");
    this.restCall.fetchEmployee().subscribe(EmpData=> {
      console.log("employee Data is :"+EmpData);
     this.dataSource.data=EmpData as Employee[];
    } ,error=>{
      console.log(error);
    });; ;
  }
}

“这是由角度cli自动生成的数据源表Ts文件”

import { DataSource } from '@angular/cdk/collections';
import { MatPaginator } from '@angular/material/paginator';
import { MatSort } from '@angular/material/sort';
import { map } from 'rxjs/operators';
import { Observable, of as observableOf, merge } from 'rxjs';
import { Employee } from '../shared/employee.model';
import { RestCallService } from '../rest-call.service';

// TODO: Replace this with your own data model type
export interface DataTableItem {
  id : number;
  name : string;
  emailId : string;
}

// TODO: replace this with real data from your application

export class DataTableDataSource extends DataSource<DataTableItem> {
  data: DataTableItem[];
  paginator: MatPaginator;
  sort: MatSort;

  constructor() {
    super();
  }

  /**
   * Connect this data source to the table. The table will only update when
   * the returned stream emits new items.
   * @returns A stream of the items to be rendered.
   */
  connect(): Observable<DataTableItem[]> {
    // Combine everything that affects the rendered data into one update
    // stream for the data-table to consume.
    const dataMutations = [
      observableOf(this.data),
      this.paginator.page,
      this.sort.sortChange
    ];

    return merge(...dataMutations).pipe(map(() => {
      return this.getPagedData(this.getSortedData([...this.data]));
    }));
  }

  /**
   *  Called when the table is being destroyed. Use this function, to clean up
   * any open connections or free any held resources that were set up during connect.
   */
  disconnect() {}

  /**
   * Paginate the data (client-side). If you're using server-side pagination,
   * this would be replaced by requesting the appropriate data from the server.
   */
  private getPagedData(data: DataTableItem[]) {
    const startIndex = this.paginator.pageIndex * this.paginator.pageSize;
    return data.splice(startIndex, this.paginator.pageSize);
  }

  /**
   * Sort the data (client-side). If you're using server-side sorting,
   * this would be replaced by requesting the appropriate data from the server.
   */
  private getSortedData(data: DataTableItem[]) {
    if (!this.sort.active || this.sort.direction === '') {
      return data;
    }

    return data.sort((a, b) => {
      const isAsc = this.sort.direction === 'asc';
      switch (this.sort.active) {
        case 'name': return compare(a.name, b.name, isAsc);
        case 'id': return compare(+a.id, +b.id, isAsc);
        case 'emailId': return compare(a.emailId, b.emailId , isAsc);
        default: return 0;
      }
    });
  }
}

/** Simple sort comparator for example ID/Name columns (for client-side sorting). */
function compare(a, b, isAsc) {
  return (a < b ? -1 : 1) * (isAsc ? 1 : -1);
}

“请告诉我，如果您有任何解决方案”