AmazonSecurityTokenServiceClient和AssumeRoleAWSCredentials都是AWS SDK中用于实现角色假定的类。它们之间的区别在于，AmazonSecurityTokenServiceClient.AssumeRoleAsync返回使用角色假定后的临时凭证，而AssumeRoleAWSCredentials返回的是用于调用AWS API的临时凭证。

以下是一个使用AmazonSecurityTokenServiceClient.AssumeRoleAsync实现角色假定的示例：

public async Task AssumeRoleAsync()
{
  AmazonSecurityTokenServiceClient stsClient = new AmazonSecurityTokenServiceClient();

  AssumeRoleRequest assumeRoleRequest = new AssumeRoleRequest
  {
    RoleArn = "arn:aws:iam::111111111111:role/MyRole",
    RoleSessionName = "MySession"
  };

  AssumeRoleResponse assumeRoleResponse = await stsClient.AssumeRoleAsync(assumeRoleRequest);

  Credentials credentials = assumeRoleResponse.Credentials;
  // Use the temporary credentials to make AWS API calls
}

以下是一个使用AssumeRoleAWSCredentials实现角色假定的示例：

AssumeRoleAWSCredentials assumeRoleCredentials = new AssumeRoleAWSCredentials(
    roleArn: "arn:aws:iam::111111111111:role/MyRole",
    roleSessionName: "MySession");

// Use the temporary credentials to make AWS API calls