ASP.NET Core API中的模型绑定始终为null

我有一个简单的ASP.NET Core API，可使用javascript的fetch方法调用。但是SearchString属性类型的输入变量模型始终为null。

由于某种原因，当我使用Postman调用它时，它可以按预期工作，但不能从javascript中调用它。

API：

[HttpPost]
public async Task<IActionResult> Search([FromBody] BasicSearchModel model)
{
    ....
}

BasicSearchModel.cs：

public interface IBasicSearchModel
{
    string SearchString { get; set; }
}

public class BasicSearchModel : IBasicSearchModel
{
    public string SearchString { get; set; }
}

JavaScript：

const WebRequest = async (controller = "API", action, query = {}, data = {}, method = RequestMethods.POST, mode = RequestModes.SameOrigin, creds = RequestCredentials.SameOrigin, headers = {
        "Content-Type": "application/json",
        "Accept": "application/json"
    }) => {
    const GetUrl = () => {
        var result = "/" + controller + "/" + action;
        var i = 0;

        for (var q in query) {
            if (i === 0) {
                result += "?";
            } else {
                result += "&";
            }

            result += q[0] + "=" + q[1];

            i++;
        }

        return result;
    }

    await fetch(GetUrl(), {
        method: method,
        headers: headers,
        // @ts-ignore
        credentials: creds.toLowerCase(),
        // @ts-ignore
        body: data,
        // @ts-ignore
        mode: mode.toLowerCase(),
    })
        .then(response => {
            if (response.ok) {
                return new RequestResult(response);
            } else {
                throw new Error("Request failed: " + response.status + "; " + response.statusText);
            }
        })
        .catch(error => {
            throw new Error("Error: " + error.statusText);
        });
};

JavaScript调用：

var result = await WebRequest("Identity", "Search", null, { SearchString: str });

要求：

标头：接受：“ application / json”，“ Content-Type”：“ application / json”

方法：“ POST”

身体：SearchString在： “测试仪”