在SelectInput.js（Select利用）中，图标呈现如下：

<IconComponent className={classes.icon} />

通过自定义图标的方式，您忽略了SelectInput传递的 className 属性，这会阻止它正常运行。

以下是自定义图标的几个示例。第一种情况直接使用图标 ( IconComponent={ExpandMoreIcon}) 而不将其包装在另一个函数中。第二种情况显示了更接近您尝试做的事情（支持在其上指定您自己的类）。虽然className是SelectInput当前尝试指定的唯一属性，但我认为您最好通过所有props：

const iconStyles = {
  selectIcon: {
    color: "green"
  }
};
const CustomExpandMore = withStyles(iconStyles)(
  ({ className, classes, ...rest }) => {
    return (
      <ExpandMoreIcon
        {...rest}
        className={classNames(className, classes.selectIcon)}
      />
    );
  }
);

这是完整的示例：

import React from "react";
import PropTypes from "prop-types";
import { withStyles } from "@material-ui/core/styles";
import InputLabel from "@material-ui/core/InputLabel";
import MenuItem from "@material-ui/core/MenuItem";
import FormControl from "@material-ui/core/FormControl";
import Select from "@material-ui/core/Select";
import ExpandMoreIcon from "@material-ui/icons/ExpandMore";
import clsx from "clsx";

const styles = (theme) => ({
  root: {
    display: "flex",
    flexWrap: "wrap"
  },
  formControl: {
    margin: theme.spacing.unit,
    minWidth: 120
  },
  selectEmpty: {
    marginTop: theme.spacing.unit * 2
  }
});
const iconStyles = {
  selectIcon: {
    color: "green"
  }
};
const CustomExpandMore = withStyles(iconStyles)(
  ({ className, classes, ...rest }) => {
    return (
      <ExpandMoreIcon
        {...rest}
        className={clsx(className, classes.selectIcon)}
      />
    );
  }
);

class SimpleSelect extends React.Component {
  state = {
    age: "",
    name: "hai"
  };

  handleChange = (event) => {
    this.setState({ [event.target.name]: event.target.value });
  };

  render() {
    const { classes } = this.props;

    return (
      <form className={classes.root} autoComplete="off">
        <FormControl className={classes.formControl}>
          <InputLabel htmlFor="age-simple">Age</InputLabel>
          <Select
            value={this.state.age}
            onChange={this.handleChange}
            inputProps={{
              name: "age",
              id: "age-simple"
            }}
            IconComponent={ExpandMoreIcon}
          >
            <MenuItem value="">
              <em>None</em>
            </MenuItem>
            <MenuItem value={10}>Ten</MenuItem>
            <MenuItem value={20}>Twenty</MenuItem>
            <MenuItem value={30}>Thirty</MenuItem>
          </Select>
        </FormControl>
        <FormControl className={classes.formControl}>
          <InputLabel htmlFor="age-simple">Age</InputLabel>
          <Select
            value={this.state.age}
            onChange={this.handleChange}
            inputProps={{
              name: "age",
              id: "age-simple"
            }}
            IconComponent={CustomExpandMore}
          >
            <MenuItem value="">
              <em>None</em>
            </MenuItem>
            <MenuItem value={10}>Ten</MenuItem>
            <MenuItem value={20}>Twenty</MenuItem>
            <MenuItem value={30}>Thirty</MenuItem>
          </Select>
        </FormControl>
      </form>
    );
  }
}

SimpleSelect.propTypes = {
  classes: PropTypes.object.isRequired
};

export default withStyles(styles)(SimpleSelect);

这是 Material-UI 添加到图标NativeSelect.js的样式（可以在其中导出样式以便Select.js可以重用它们）：

  icon: {
    // We use a position absolute over a flexbox in order to forward the pointer events
    // to the input and to support wrapping tags..
    position: 'absolute',
    right: 0,
    top: 'calc(50% - 12px)', // Center vertically
    pointerEvents: 'none', // Don't block pointer events on the select under the icon.
    color: theme.palette.action.active,
    '&$disabled': {
      color: theme.palette.action.disabled,
    },
  },
  /* Styles applied to the icon component if the popup is open. */
  iconOpen: {
    transform: 'rotate(180deg)',
  },
  /* Styles applied to the icon component if `variant="filled"`. */
  iconFilled: {
    right: 7,
  },
  /* Styles applied to the icon component if `variant="outlined"`. */
  iconOutlined: {
    right: 7,
  },

绝对定位将图标保持在 的可点击区域内Select。如果没有绝对定位，图标将导致组成 Select 的不同元素发生移动。单击图标应该继续工作的位置，但元素移动的方式导致 Select 的整体布局不再有意义。