您的型号以字符串表示形式发布，不是 JSON 。

await modelContent.ReadAsStringAsync（）将以数组表示法返回一个字符串： [object Object] 。

Your model gets posted in its string representation, which is not JSON.
await modelContent.ReadAsStringAsync() will return a string in array-notation: [object Object].

您必须自己处理JSON序列化。

You'll have to take care of the JSON serialization yourself.

替换

data.append("model", { "TenantId": this.state.TenantId, "TenantUrl": this.state.TenantUrl, "TenantPassword": this.state.TenantPassword });

with

data.append("model", JSON.stringify({ "TenantId": this.state.TenantId, "TenantUrl": this.state.TenantUrl, "TenantPassword": this.state.TenantPassword }));