Error on ComputeSignature when changing from SHA256 to SHA1

[Cae2]

Hello,
I need to use a service who still dont accept SHA256 Algorithm.

the following works when I execute:

var docXml = new SignedXml(xml);
...
docXml.SignedInfo.SignatureMethod = "http://www.w3.org/2000/09/xmldsig#rsa-sha1";
docXml.ComputeSignature();

but the Xml Reference stills in SHA256. If I change the Reference to also use SHA1 with:

var refer = new Reference { Uri = "xxxxx" };
...
refer.DigestMethod = "http://www.w3.org/2000/09/xmldsig#rsa-sha1";

the ComputeSignature() shows the error:

Exception has occurred: CLR/System.Security.Cryptography.CryptographicException
An exception of type 'System.Security.Cryptography.CryptographicException' occurred in >System.Security.Cryptography.Xml.dll but was not handled in user code: 'Could not create hash >algorithm object.'

can someone give me some advices to change all algorithms in that xml signature to SHA1?

thanks!

Edit:
ops, forgot to mention that I'm using ASP.NET Core 2.0, also tried with the last
System.Security.Cryptography.Xml 4.5.0-preview1-25914-04

[Cae2]

solved, I was using a wrong algorithm

changed:

refer.DigestMethod = "http://www.w3.org/2000/09/xmldsig#rsa-sha1";
for
refer.DigestMethod = SignedXml.XmlDsigSHA1Url;

[leofonsilva]

solved, I was using a wrong algorithm

changed:

refer.DigestMethod = "http://www.w3.org/2000/09/xmldsig#rsa-sha1";
for
refer.DigestMethod = SignedXml.XmlDsigSHA1Url;

Hello, I would like to know how you created your signature class, because I tried everything, did what you said and did not work, I can not sign xml using SHA1 in Core